Hi there
I think it's pretty normal behaviour for 3-4 year olds not to want to play with the little ones.
I completely agree with JazaHMum.

I wouldn't worry about it hun. I remember when my DS started kindy and I use to quiz him every night about who he played with as I was so worried that he would have no friends! Is there any chance you could invite one of the play ground mums around for a coffee? That way your DD will have one on one play time. Either way I wouldn't worry she is so young still.