I'm a long way from having kids (or so I hope, that last chick I beat is starting to get a bit big ) but I would treat it as a kind of playful little thing whilst they're a child, never push it, and once they were a teenager, I would let them do it. Then after they'd been at it for a decent while (like a year proper I'd say), I'd go see em fight/spar a couple of times & make an assessment of how far they could go. Unless I realized they were gonna be somethin really special, I would do my best to make sure they kept it as an amateur hobby