When my now 6 foot son was younger, I had a crossbar seat for him. However, coming down a hill one day, he decided to copy me and reached for the brake...
Luckily, after a checkup at local A&E [remember those] we were both fine. Psychologically, I prefer the crossbar designs where the child is in front and visible. On my cycles, I've seen too many kids in rear seats sitting or behaving in *undesirable* ways. However the crossbar designs always has the risk that a young child can grab the brakes.
The only answer was to get him off my bike and steering his own wheels - slower but safer.