SFO - we ride the little two coach people mover train that runs between the terminals and the rental car building. The kids get a thrill out of riding a train and seeing different planes (777, 747, 757etc) on the tarmac is icing on the cake.

SIN - T2 had a kid's play area near the F gates Transit desk. There are many ponds and small gardens scattered through both T1 and T2, that the kids kept busy.

BOS - others have mentioned the Kid port already.

SJC - Kidport mentioned by others already.