I dont know anything about you, but there are 2 issues here one is the religious one and the other is the safety issue.
Even if your child is not relgious at this time, you need to get them into something productive. Better to work at McDonalds on Shabbos than have them doing drugs or some other dangerous and harmful activity. At least they will be safe there. You can always do tshuva from working at McDonalds on Shabbos. You cannot do Tshuva if you O’Ded on Heroin