Due to the amount of activity I usually do at the park I normally either eat nothing and have a decent meal before I leave, or if I feel the need for something, or want to treat myself have a little bit of a snack before I do so later on. However, to answer your question, if by the chance you find a place that doesn't have a kids menu, yes, they should be able to order from the adult menu. If they do not have a kids menu online, I would imagine the location itself should have a kid's menu though.
Also, if you plan on doing the QS plan, you might have well be going for the things that cost the most to get the best bang for your buck, if not, you might as well not be doing the QS dining plan at all. You can easily go over $23.99 ordering the right food and using all the options you receive with the QS dining plan, but if you do not a select the right items, or do not use them all, you'll barely reach the mark, or be low the mark and if that's the case it won't be worth it. However, at the same time, some people like the idea of playing a flat fee and not having to worry about paying for here, or there.