I think its a bad idea to do it and not let them know. If your kids are teenagers then when they go out with friends, etc. and in school it is to easy to get carby snacks. You might think there eating healthy at home but away from home there not. So they will probably gain even more weight.

If you want to do it then let them know so they won't go eat carby stuff.

These days its really hard for a teenager who is overwieght. I bet you they would love to loose wieght, so just ask.