Are they being held? Yes.
The Telegraph article you quote has the key facts. This article in Le Monde (Google Translate) has more details.
Are they children? Technically yes.
The Le Monde article says they were students at the Lycée Jean-Rostand. In France a lycée is for students aged from 15 to 18. So yes they are likely to be mostly minors, but still older than the word "children" generally implies. Le Monde states that out of the whole 190 people arrested in the Yvelines Department the youngest was 12 and the oldest was 20, but does not give exact details for the arrests in the video.
At gun point? Almost certainly not.
The Telegraph describes the police as "armed", but the videos do not seem to show any drawn guns. Some of the police are holding batons, but that seems to be the limit of the threat. The uniforms saying "Police" indicate that this is the "Police Nationale". In this particular case, several outfits are marked CSI 78, which stands for "Compagnie de sécurisation et d'intervention", a specialized riot control corps (Google Translate) within the police, for Paris and close suburbs. The "78" indicates these men belong in the Yvelines departement (#78). According to that page the riot police carry batons and shields as standard, but firearms are available to them if circumstances require it. The Le Monde article describes the authorities exercising restraint the previous day, so it seems unlikely that they would have issued firearms for a planned operation to detain a crowd of teenagers.
For peacefully protesting? No.
The following is from the Google Translate of the Le Monde article:
The escalation of violence continued the next day. The gathering of some 300 students in front of the Lycée Jean-Rostand quickly degenerated with "barricades fires" followed by "throwing rocks and pebbles on the police," says the same witness . In parallel, a small group entered the surrounding homes to steal a dozen bottles of gas, which would have been thrown into a trash. "But, fortunately, they did not explode," says the inhabitant, who saw the young people then heading to Val-Fourré.
The following day the students attempted to demonstrate again, but this time the police were ready for them and arrested everyone present. That is when this video was shot. From the Google Translation of the article:
Thursday morning, the first bins were burned around 9 am in Saint-Exupéry before two vehicles ignited in the parking of the ice rink, 300 meters from the hotel. "The high school students wanted to block the entrance to the school, located at the end of a dead end, but they quickly turned around, because the police, who was hiding around, was waiting for them, details a resident.
