Y.B.
Probiotics are great! You do want to find a high quality one, and typically should be kept in the refrigerator. I like Children's Probiotic Powder by Integrative Therapeutics. Something else to consider is how much is she drinking? Often times, constipation in children is related to not consuming enough liquid (ideally water or diluted juice). That being said, while concentrated juice can sometimes act as a laxative, too much/daily concentrated juice can cause constipation. Finally, a lot of things that we consider to be great in helping constipation in adults can be counter intuitive for children. For example, the pectin in apples and grapes, as well as certain high fiber foods can cause constipation in children... just something to think about.
Adding more liquid to my son's diet worked wonders for us. Good luck!