At my son's fourth birthday party recently I had a buffet of grilled cheese sandwiches- regular american cheese on whole wheat for kiddos and bland palattes, provolone, fresh tomato and basil on whole wheat, and mimi's grilled cheese (google for the recipe or e-mail me and I'll send it to you if you want it)- a sandwich with parmesan crusted bread and four different kinds of cheese in it. We had veggies and dip, a fruit salad and cake to round out the spread. It was cheap and everyone seemed to like my take on a kid favorite.
For my daughter's upcoming birthday I'm thinking of doing a buffet of different kinds of quesadillas (we have a love afair with cheese here as you can see).
I've also done a party with build your own panini (if you don't have a press you can use a George Foreman or a skillet with a cast iron skillet on top of it to mush down the sandwiches).
When my son was two one thing we had on the menu was pb&j sushi- make the sandwiches, roll them flat with a rolling pin, roll them up and cut them california roll-style.
Another thought- a chicken finger buffet- some with breading, some naked, some dipped in wing sauce, with a variety of sauces for people to dip them in. Add tortillas and some fresh chopped veggies and people could make wraps too.
Good luck!