Not sure about the lunch time question, but most restaurants are pretty good about letting anyone order off of kids menus.The adult menu does have many things that kids would like,and the kids menu does as well (plus it is much cheaper).
If I remember correctly, dinner begins at 4:00. Anytime before that is lunch. But I also don't remember there being a price difference between lunch and dinner at the Prime Time. Am I just forgetting?
There are less expensive items on the lunch menu that aren't available on the dinner menu. We were seated at 3:55 and given the choice of which menu we preferred, and DH chose the dinner menu... I think the portions are larger.