I would cast my vote for Kodak Portra UC - even the 400 version is very fine grained. I shot the Fall colors in Vermont with it (35mm) and have been very pleased with the enlargements from it, though none went past 11X14. But if they are scanning them I assume the prints would just be the standard 4X6, or are they scanning the negs/chromes directly? If chromes can be used, Kodak has some "enhanced saturation" film that would show off the colors of the costumes well.

Whatever you go with, I would also suggest a polarizing filter, which will make those colorful costumes really "pop." And the fill flash you mentioned will undoubtedly be a good idea, especially if it is a sunny day, or a cloudy day, or in the shade....(you get the idea) And since it is four-legged children, if I were saddled with the project I would go with the 400asa film. Sounds like a fun day!