I always recommend Le Cellier over San Angel. The food is much better and the servers there are very nice and helpful. When my DH and I went there last summer, there were plenty of children in the restaurant. Also, I've had heard & seen pictures of the "Moose" dessert that seems to be very popular with the kiddos. Its very cute. I think it would be a great choice for your group.
On a side note, the inside of San Angel is very neat...I just think the food and service is sub-par and not worth the money. And I wouldn't think too much kid friendly food. However, we usually just go inside the temple and walk around to cool off and look in the "marketplace" area, without eating at San Angel.