Children are only required to fast when they attain puberty. Before that, it is not compulsory. This applies to all forms of worship.
The popular belief, especially amongst Arabs that children have to fast when they are 10, and according to some retards, that children have to fast when they are 7 is wrong.
Fasting is a form of worship that a person chooses to do for Allah. It is not something that can be forced, even if a child is mature.
Many parents who opposed the ridiculous and barbarous rule of the Taliban themselves hold the Taliban mentality when it comes to their children fasting.
It is a choice. Your daughter/son will do it for Allah because it is for Allah. Not for you.