Friends and I have done this before. At counter service restaurants you can get peanut butter and jelly sandwiches (one of my favorites) as a meal with chips and a drink, works out to be a good deal. Never had a problem with it.
Probably obvious but if you're going to a buffet type restaurant you pay the adult price regardless of what you eat.
Also, if you have a friend to split a meal with the adult size portions are generous and most places will split the plate for you in the kitchen (bring out the pasta dish on two plates so you don't have to divide it).
Also there have been a couple of places that offered "half portions" of pasta type dishes or sometimes offering "lunch portions" at dinner. Not everywhere, not always but it doesn't hurt to ask.
As indicated, the children's menu doesn't always offer the most variety.