Sounds like OP did neither. While I dont trust cops at all, statistically, it would have been better to make the call because they more often than not don't shoot people. And actually, yes, bullets would solve the child abuse problem, just not in a way anybody would want. Can't abuse a child if you're dead.
I'd absolutely roll the dice for the child's safety over the piece of shit parents life.
It's negotiating 101. Open with the hoop dream so the next option isn't near as bad. If you get your first option, you win, and if you get the second option, you win a little less.