It seems that for special occasions kids always have a way of ruining the style before they even get to the special event.
I recommend buns for girls only because it's practical and if they sweat out their hair then it's still tamed. When you deal with kids, it's only natural to want to give them curls and straighten their hair, but you don't want to waste money on something that's only going to last for the day, if even that long. Texture also has alot to do with what style you choose. I personally feel that a bun works for any texture of hair from fine-coarse. If the hair is fine, you make need to altar the bun (It may need teasing to give it more volume, and fill in the gaps).
Sometimes little girls wear braided hairstyles. A bun can still be achieved with certain braided styles as well.
If you are seeking to soften your little girls curl pattern, a relaxer or keratin is not the answer. I don't believe in applying chemical straighteners to kids hair but I do believe in a pressing comb, blow dryer, or flat iron.
If the texture of the hair is curly but on the fine side, all you need is a good blow dryer and maybe a flat-iron. A heat protectant is recommended as well. I like Paul Mitchell "super skinny" and also Kerastase' "elixir ultime". For edge control try Tigi Bedhead's wax stick. It's a miracle worker.