On the Gateway Pundit, this claim by Chaldean-American businessman Mark Arabo is posted:
“There is a park in Mosul, where [ISIS] they actually beheaded children and put their heads on a stick and have them in the park,”
Is this true?
There are multiple news outlets reporting ISIS has beheaded children quoting the same source, Mark Arabo, and there is one source (catholic.org) that shows disturbing 'evidence' that seems to be misattributed, actually stemming from Syria. There is reason to be skeptical, as the recent news about the call for female genital mutilation by ISIS and some other grand claims were wrong too.
If you feel like nobody would be capable of such acts, you could watch the VICE documentary about ISIS and be convinced otherwise within 9 minutes.
This report from catholic.org holds images that ChrisW in the comments showed to have been published before, and seem to stem from the civil war in Syria. The veracity of any report is hard to determine. I would urge you to consider before following them, as a picture will never hold the full contextual information (as has become obvious within the comments to this answer).
It does seem like ISIS modus operandi to behead. They did so in a widely reported action where 50 Lebanese soldiers were murdered and beheaded, a video of which emerged. Again, I advise caution gathering information on this subject, although this one seems quite trustworthy.
The claim quoted in the question is most likely untrue. There is strong circumstantial evidence that indicates it is not true. According to thegospelcoalition.org:
The source of the claim comes from Mark Arabo in an interview with CNN. Arabo says that ISIS is “systematically beheading children” and that “there is a park in Mosul in which heads of beheaded children are put on a stick.”
Arabo is businessman in San Diego, California who is described as a “spokesperson for the Iraqi Chaldean community in San Diego County.” Arabo has been instrumental in promoting House Resolution 663, a resolution that expresses an “urgent need to protect religious minorities from persecution.” While he is a passionate spokesman for Iraqi Chaldeans and obviously sincere in his convictions, there is reason to question whether his claim about beheaded children is true.
There is no doubt that ISIS is persecuting the Christians in Mosul and other areas of Iraq. But almost all have already fled the city and the few that remain are continuing to leave the area. There are, however, journalists from Iraq and Western news agencies still in the city. Why have none of them taken photographs of these atrocities, or even reported on their occurrence? Why have such stories not been reported by the Christians who have fled to the cities controlled by the Kurds?
If Arabo was able to get news of such atrocities in San Diego, why has no one else heard stories of this ongoing tragedy?
Fueling the speculation has been websites, like Catholic Online, that purport to have pictures of children beheaded by ISIS. (The images on the site are extremely graphic and disturbing. I’d advise not looking at them.) Catholic Online, which is not officially connected to the Catholic Church, has only one picture that could be of a beheaded child. But there is no way to know whether it is real or whether it occurred in Iraq. There is no source or context for the photos and the story is credited to “News Consortium.”
One of the pictures that Catholic Online includes — and that has become ubiquitous on social media — shows a baby with three rifles pointed at his head (see image above). While the image is outrageous, it was not a photo taken of ISIS in northern Iraq.
The photo originally appeared online April 11, 2014 on the Facebook page of a person from Yemen. Numerous people on that page attest that the clothes the child is wearing are obviously Yemeni. A few days later, though, the image started popping up on pro-Syrian Army websites claiming that it was an Armenian child who was taken by Syrian rebels. Whatever the original context for the photo, we know based on the date alone that it was not recently taken in Mosul or northern Iraq.
While it is possible that children are being beheaded by ISIS in Iraq, there is currently no credible evidence to support that claim.
The mainstream news stories, reporting people who were there, are of Christians having been expelled from Mosul, not of Christian children being murdered there.
Chrétiens d'Irak : rester ou partir? says (italics are the original french, followed by my translation from French to English):
Le cardinal Philippe Barbarin, archevêque de Lyon, revient d'une visite en Irak, où il a rencontré des chrétiens récemment chassés de Mossoul par les djihadistes.
The cardinal Philippe Barbarin, archbishop of Lyon, returns from a visit in Iraq, where he met christians recently chased from Mosul by jihadists.
« Tout d'un coup, le prétendu calife de Mossoul a dit : "À partir du 18 juillet à 12 h, plus un seul chrétien dans la ville de Mossoul. Les maisons ne sont plus à vous, ce n'est plus votre ville, vous devez être partis". »
Suddenly the pretended Calif of Mosul said, "Starting from July 18 at noon, not a single Christian in the town of Mosul. The houses are no longer yours, it's no longer your town, you must be gone."
Les chrétiens ont fui à la hâte, emportant leurs enfants et leurs biens les plus précieux, explique l'archevêque. Cependant, aux postes de contrôle, à la sortie de la ville, « on a pris la voiture et on les a laissés partir avec le minimum de vêtements et rien rien rien dans les poches, rien du tout ».
The christians hurriedly fled, taking their children and their most precious goods, explained the archbishop. However at the control points, at the exit from the town, "the took the car and they let them go with the minimum of clothes and nothing nothing nothing in the pockets, nothing at all."
Le cardinal souligne que pas un seul chrétien n'a perdu la vie, contrairement aux chiites, qui ont été massacrés en grand nombre par les sunnites. « C'est très surprenant. J'ai vu le patriarche leur dire : "Arrêtez de vous plaindre; pour les musulmans chiites, c'est pire que pour vous." »
The cardinal underlines that not a single Christian lost their life, unlike the shiites who wre massacred in large numbers by the sunis. "It's very surprising. I saw the patriarch tell them "Stop complaining; for the shiite moslems, it's worse than for you."
This story gives an example of why the Christians left.
L'EIIL multiplie les exactions dans sa conquête des villes irakiennes, et chaque jour apporte un lot de témoignages effrayants. Ainsi, Bassam, qui a toujours vécu à Mossoul, vient juste de se réfugier en Turquie. «Tout ce que je possédais, l'État islamique l'a pris», raconte-t-il. «Ils sont venus chez moi et m'ont dit : «Tu vois ta femme, on revient avec un couteau, on lui ouvre le ventre, et on tue le bébé». Alors j'ai pris ma famille et je suis parti tout de suite», raconte cet homme, qui vivait avec sa femme enceinte, ses deux enfants en bas âge et ses parents. «Ils détruisent tout ce qui est chrétien, même les cimetières».
ISIL multiples its exactions in its conquest of Iraqi towns, and each day brings frightening testimonies. Thus, Bassam, who has always lived in Mosul, has just come for refuge in Turkey. "Everything that I had, the islamic state took it", he said. "They came to my house and told me, "You see your wife, we're coming back with a knife, we open her belly, and we kill the baby." So I took my family and I left right away." tells this man, who lived with his pregnant wife, his two young children, and his parents. "They destroy everything that's christian, even the cemetaries."
As of today the only source for stories found on the first few pages of https://www.google.com/search?q=behead+children are quoting Mark Arabo, and he doesn't justify his statement on http://www.markarabo.com/ (he only provides a link to the same CNN interview) therefore IMO his specific statement quoted in the OP and question on CNN, i.e. that children are beheaded, systemtically or otherwise, in Mosul, is uncorroborated.
Furthermore, on his blog dated July 22, Mark Arabo wrote,
Recently, images have gone viral that show a "Red Death Stamp" being placed outside of the homes of Iraqi Christians. This stamp is a reminder to all Christians that once the Holy Month of Ramadan ends, ISIL knows who you are, and your life is in their hands now. Because of this, 95% of Christians have left Mosul with about 5% remaining as converts to Islam. The red badge shown carries echoes of the Holocaust and the yellow badges placed on the arms, and outside the businesses of Jewish individuals. It is a frighteningly clear omen of what the future carries for these marked Christians.
If his numbers are correct, that agrees with the previous assessment that no significant number of Christians were killed (instead, they were warned to flee, and they fled).
There's seems little doubt that people who didn't flee would have been killed. For what it's worth there is good evidence that they were systematically warned to flee instead of or before being killed.