Make sure your children understand the rules. If they are small you can program in the numbers that they will need so that it will display who is calling them. Tell them not to answer calls from numbers that they don’t recognise. 

Make rules for time spent talking on the phone so you won’t be surprised by an astronomical phone bill.

Check this regularly so you know that they are keeping to the boundaries. 

Most schools ban mobile phones in the classroom and will confiscate them if found. Make sure your child is aware of the rules and the consequences