We’re not just talking about a person who has moved on from a long-term relationship, though. PP’s are normalizing it because they’re only focusing on the aspect of him dating so soon, etc.
But you’ve described a situation where your ex has become an absentee father, which reflects very poorly on his character and integrity as a human being. It’s one thing to move on from your ex, but it’s anotger thing entirely to move on from your kids, and let them down again and again.