(Bolding is the submitter’s)
Maybe not inexperienced but could be a possible unsafe environment. I'm childfree, I don't shove my opinions down other people's throats like if you want kids have kids, but I absolutely despise them. They fill me with inexplicable rage I cannot explain. And because I know I can't trust myself not to snap and lash out at a kid, I don't put myself in situations where I'm around kids. I don't babysit, I don't go to kid filled areas, I don't even watch my niece for a longer span of time than 5 minutes because everything about kids infuriates me. I'm not a violent person but something about kids inherently makes me want to hurt them. I never would, because I know myself enough to know not to put myself in situations where I could risk getting that angry.
So, someone who hates kids being forced to look after a kid could end in that kid getting hurt in a snap second moment of rage. Not that that is at all okay, but it can happen without the intention of hurting the kid upon agreeing to watch the kid. OP shouldn't have called the police, but if he felt himself getting that angry he should have made arrangements for the kid to go elsewhere himself, without police intervention. Should've called mum and gotten her to come get nephew or taken him there.