B.,
I just had a 4th birthday party with 9 little girls (ages 14 months - 5 years). We had a Hawaiian theme. Each girl got a lei and a hula skirt when they came into the house. They also did a treasure hunt at the end of which they found Hawaiian fans. I ordered everything from Oriental Trading.
I served hotdogs, turkey burgers and blackbean burgers, pasta salad, fresh fruit and potato chips. We had lemonade to drink and cake and ice cream afterwards, of course. it was very simple, but the kids really didn't care about the food. They cared about the piñata, treasure hunt, and other games. I think that you can do ANYTHING and as long as there is a cake at some point, they'll be content.
Last year I did an elaborate 'Little Mermaid' theme replete with shell shaped pastries stuffed with chicken salad, a watermelon cut to look like a crab (and filled with fruit), and a homemade cake that looked like a rock surrounded by ocean waves with an Ariel doll perched atop it. NOBODY CARED. The kids ate NOTHING except cake. They just wanted to play. More kids ate this year, but were not even noticing the fresh pineapples I had decorating the table, nor the grass skirt around it with pretty hybiscis flowers.
Worry not about the food. They will be content with hot dogs and chips. It is we adults who worry about themes and fussy foods. Offer unlimited beverages and chips and they will think you walk on water... :)