From day care to 2nd grade my favorite FF kids lens has been my 24-105mm zoom. Wide enough to get the whole class at Halloween, but still get in tight on my little monster. IS helps indoors. Of course the 70-200mm is important for sports, school play etc, but when mixing it up, I need the versatility of the zoom.
I had an 85mm f1.2 for a while, and I don't think I ever got 1 sharp photo of a child. The kids just move too fast.