I would rather my children have a cell phone that I can monitor than a prepaid one that I cannot control.
I am not of the opinion that cell phones are inherently dangerous, especially if you teach your children how to use them properly. Even better if you monitor them and ask them who’s numbers they are.
2morecents, you reference “kids” in their 20s – they are adults at that point. Often married with children of their own. You also reference chinuch – chinuch is not just about avoiding anything that possibly could become treif (otherwise, we wouldn’t eat cheese and hamburgers lest we combine them). Chinuch is about teaching your children the proper way to use communication tools.