According to the Law of Attraction, whatever you give your attention to, you attract more of.
That means there's effectively no such thing as NO... There is only YES!
If you tell your child, "No, you may NOT have any candy! It's BAD for you! You shouldn't eat that junk," then you're essentially saying YES to candy by focusing so much attention on it. The more you resist it, the more you're thinking about it, which guarantees more and stronger thoughts about candy (or whatever you're resisting).
The key to happiness in a YES-based universe is to give very little attention to what you don't want, and lots of positive attention to what you do want.
If you truly believe your child is better off without something, then look for something else to say YES to — preferably something even more attractive.
Remember that Life Energy and Unconditional Love are more attractive than any thing.