Since pageants are usually somewhat formal affairs, it usually works well to wear the hair in an upsweep. Since we are speaking of a child, it would work best not to keep it too old fashioned or mature, so leave some front pieces falling on the face and make it young. It may work to use a "youthful" hair embellishment.
I would say keep it simple and appropriate for her age, there will be costume changes and so you want something that looks good with all the outfits, too.