I think it depends on the restaurant AND your server.
We were encouraged to order a childs dessert at the Rainforest one year as we were very full. When it turned up (we shared) it was HUGE, no way was it a kids portion, but thats what we were charged for!!
However, I have also read on the US boards that since the dining age at Disney has been lowered, many on property restaurants are taking a harder line with "adults" ordering a chillds meal - and charging double for them!
I think the best tactic is to ask for a smaller portion of an adult entree. They may well reduce the cost...I am positive most off site places would if the situation was explained. They can also "split plates" so if you have two light eaters they will happily serve one entree split between two plates.
Its also worth bearing in mind that many restaurants (especially the resort ones) will usually fix something that is not on the menu. So, if your light eaters want a plain simple meal like chicken breast and salad, or a cheese sandwich they will often be able to do that.
I dont think you will find that there is a "one rule" answer to this question. Unfortunately, you are going to have to explain at every establishment. The idea of a printed card is a good one and might be worth doing.