My daughter had the same type of rash except she also had it on her elbows, I cut out eggs from my diet and it seemed to help quite a bit.
If it does happen to be excema, drinking Aloe Juice could be of great help. I would consult her pediatrician first but I have several friends who's kids had excema and drinking aloe juice keeps them from having outbreaks. It also helps me control my dandruff.
I buy mine from www.marketamerica.com/pgp They have different flavors that actually taste good and can be mixed with water or juices.