I love this idea . . and have a couple of suggestions
I don't know how adventurous the little ones are, but how about . . .
Tea sandwiches . . .
PBJ tea sandwiches . . . little PBJ sandwiches, perhaps double stacked, crusts cut off, cut in triangles
Apple and cheese sandwiches . . . same presentation
Egg salad . . . you get the picture. This is all about presentation rather than trying something new.
Maybe the little ones can help assemble, or make their own.
For the petit four part, you could make mini cupcakes, mini brownies. . . mini anything decorated quaintly. Or, how about this . . . a chocolate fondue with cake and fruit? It wouldn't have to be too elaborate - it could even be just a small bowl of icing or something for the kids to dip into. You might even try some cheese for the more adventurous.