The biggest thing I would probably question about a rear rack mounted seat is the weight capacity. I would think that you would be limited by the capacity of your rear rack (mine is 55 lbs). The seat itself has to be somewhere between 5-7 lbs, so you could probably put a 48 lb kid on there. An average 3 yo is around 30 lb, but with clothes, gear, and if he/she has been hitting the Twinkies, you would have to keep an eye on that.
The odd thing about this particular seat, is it boasts a weight capacity of 100 lbs, but I don’t see any frame mounting hardware, other than what attaches to the rack. With that in mind, I would think you would be limited by the rack’s capacity and not what the manufacturer states for the seat (or the lesser of the two).