As long as they know how to talk to and explain things to a kid it should not be a problem.
This here lies the problem. If someone has never taught a kid or many kids they may not have the skill to successfully keep a kids attention or be able to have them understand what they are explaining. I would be choked if I wasted my money on an instructor and my kid(s) learned nothing.
I could teach him myself but I rather him learn things from someone certified so he doesn't pick up any bad habits I might have. Also your own kids/wife/sibling will never fully listen as well as they would to someone else.
case in point me trying to teach my wife years ago. she is horrible at "breaking the wrist" in the back cast and went crazy after my constant reminders
She will fly fish now but only if its close quarters where its just flipping out a nymph rig or dry or make me toss it out for her when we are on the lake
I am thinking about teaching my oldest to cast two handed. I feel he would benefit from using both hands over single handing. He could always use a switch rod and roll cast out a bobber setup on the lake