IMO, you go into a place, order something, and pay for it, it shouldn't matter who you are and what you order. Within reason. Going into a place and reserving an 8-person table for just yourself and nursing a single glass of tea for 2 hours is not within reason. A single adult member of a party ordering off the kids menu definitely IS within reason. Now a party of 8 adults, all ordering off the kids menu, I still would think is within reason, but after reading the quoted section below, I think maybe it's borderline. Undecided.
I understand where you're coming from but I still don't know if I can get behind it. Unless it's a buffet. Then I'm on board. But if you own a non-buffet restaurant and that's the policy you choose, then that's your prerogative. It is (or should be) a free country and nobody is (or should be) able to tell you that you CAN'T have that policy. But if you have that policy and it's something that matters to me, I'll exercise my freedom to eat somewhere else.