I don't know if you consider this dessert or not, but I would do Jello Jigglers... You can do them as heart shapes or just do any kind of shape in red, pink, and purple. Preschoolers love eating with their hands and they love the jiggliness of them.
Or you could bring sandwiches cut into heart shapes (I got a great sandwich cutter from the dollar store that cuts two hearts at a time, I still see these cutters there for sale).
Or pasta salad with strange shaped pasta or tortellini.
Or cut up fruit and make some kind of dipping sauce (preschoolers also adore dipping foods).
Or how about making chicken fingers (cheaper to just buy the skinless breast, freeze it for a few hours, then cut it in thin, long slices) and putting a sauce on the side for dipping?