One easy way to dress up hair for any type of formal or informal event is to change up the texture and add hair accessories.
If your daughter wears her hair straight for everyday, she could either add some soft volume, waves or light curls. If that doesn't work, she could wear her hair like she always does, but add a crystal encrusted headband in a color which works with her dress.
Or she could wear one side of her hair tucked behind one ear and head in place with a gorgeous hair accessory such as a crystal encrusted hair clip or a seasonally appropriate hair flower.
If her hair is long enough she could play with an accent braid as well.
Hair accessories which are embellished with shiny crystals could be a wonderful and inexpensive solution for a great dressy hair style.