While it can be smart to go vegetarian for a kids' party (they can be so finicky!), it's a pretty good bet your little partygoers will be happy to eat ground beef. So if you'd like to do a protein, consider tacos! Ground beef is a low-cost meat, homemade taco seasoning will cost you practically nothing, you can buy taco shells in bulk, and toppings like lettuce and cheese are some of the least expensive items in the grocery store.
For some extra kid appeal, set up a taco bar with lots of options. Picky-choosy-do-it-yourself is typically a hit with the younger crowd.