The principal problem is for the child because the coach is ever present in their lives. For most kids, they can go home and are no longer under the coaches' watchful eye. When you coach your own kid, you're always there urging them to do their workout. That's great if they're super motivated. But there is a risk that having the parent there always reminding them to do their workout will take the joy out of running. It can become like any other chore that the parent requires the child to do - do your homework, clean your room, do the dishes, take out the trash, go for your run. You need to have a strategy to make sure the kid runs because he/she loves it and is motivated to get better rather than just doing it to satisfy mom/dad.