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 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):
This story gives an example of why the Christians left.
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,
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.