I would trust either arrangement. Don't like seat-post mounted racks, but in this case the seat-post clamp is only providing the top attachment and the weight of the child and seat is being supported by the lower support at the rear dropouts of the bike. I used a child seat with the top support attached to clips around the seat stays and generally preferred it to using a trailer due to the closer interaction with my daughter. When she outgrew the seat she became the stoker on the back on our tandem.