I never commuted with kids, but let me join in the 'no rear seat' brigade. I rode with my kids in them, but I think I always stopped well before the weight limits of the seat. I found them to be very hard to load (you have to put the kid in before mounting your steed, and then you have to mount with 25-40 lbs of child sitting high above the rear wheel), dismount (same problem in reverse) and they do affect stability (I've never had my panniers throw themselves to one side when they see something cool -- but my kids have!)

I'd definitely advocate for either the front seat (which should improve on all of the above problems) or a trailer.

However, if you do elect to go with the rear seat, expect more wheel suckers -- riding behind a bike with one of those lets me draft from way back ;-)