Apple's Ask To Buy System can be pretty helpful if you are trying to control your kids' iTunes purchases.
In case you are not receiving Ask to Buy requests, check if you signed with correct Apple ID and also check your Family Sharing settings.
If you miss Ask to Buy request or notification, try to find and set it up in the Notification center on your Apple device.
Tip: Don't forget that not all devices support a Family Sharing feature. You need iOS 8 or later on your iPhone, iPad, or iPod touch, and OS X Yosemite or later on your Mac to set up or join a Family Sharing group.