J.H.
Judy Mac from Sally's music circle is great. http://www.sallysmusic.com/
I think that she is playing for the kids at the Milk Boy cafe in Bryn Mawr at 10:30 on Saturday if you want to check her out.
She is one of the teachers of my sons class and does parties as well. We have been to a few and she really gets the kids up and dancing...it is really cute.
I am not sure about cost, but she would probably come to the park