It’s important that your child’s legs be firm on the ground, or on a small stool while playing. So if your child is small and the legs don’t reach the ground, get a plastic (make sure it’s firm not to wobble under her legs) or wooden stool large enough for the two feet to rest on comfortably.
 
If you really want to get fancy (and spend a few hundred dollars on it), you can buy one of those adjustable stools that even have a pedal that you can press to get the right pedal pressed; the height of the stool can also be adjusted.