When do you need to stop a child from grinding his teeth at night?

You Can't: Grinding teeth is very common in children but it is not something you can control. Later as a late teen or adult plates can be made by your dentist to help prevent the grinding from doing damage.

Around 7: Grinding is quite common in children before 6. It is usually different than grinding in adults. This natural adjustment of the bite usually slows down and stops by the time the first permanent molars erupt between 6 and 7. If it persists or of it is causing excessive wear of primary molar teeth then it should be re evaluated.