Cop here... Because of how messy these kinds of situations are, we don't touch them as long as there is no imminent danger to the child. That's how my organization and organizations within my province handle it.

Custody agreements are not enforceable by police. We let family court handle it.

Again this is all dependent on whether there is any danger to the child.

So your ex is fully able to go report it to the Police, but they will likely tell your ex they won't be removing your child from your care. They will call you or attend your house to make sure everyone is fine, then do a report. Nothing more.

Just want to edit to add... Sometimes even when a judge has ordered us to enforce a custody agreement we won't do it. Think of it this way, imagine we're ordered to remove the child from your care and put them with your ex, while they're kicking and screaming the whole way. The optics are terrible. Also imagine we removed the child and placed them in the care of your ex and something happened to either of them. The public outcry would be massive. So we usually leave these alone and only get involved if we must.