I've already done this by flipping 20 kg rated QR carrier rack and zip tying a saddle to it. My kid and I have done probably 1000 miles on it without incident, he just puts his feet on the top of the forks. He's five now, I would say age range is 2-4 and a half or so, so he's too big now. This way of carrying a kid is the safest because in a crash it allows the parent to hug the child and take all the injuries, also where the kid sits is about the best you'll ever...