Of course you could just ask if he has children or you could let them know that DP told you that he has 4 DC that he is NC with. (It’s no secret if your DP knows).
Just say “DP (sorry I can’t remember if he’s your DH and I can’t go back to check without losing my post) and I were talking, generally, about the family and DP mentioned that your DP has 4 DC that don’t visit him. That’s sooo sad 😢 Why don’t they keep in contact. What happened?”
At least if they know you know they can’t deny it. They may come up with some BS excuse but you have more chance of checking out his story if you give him the chance to tell his version. Fish a bit about his D.C. “Oh, do they live near, how old are they, any D.C.” etc
You are right not to leave your D.C. with them until you know they will, at least, be safe from intentional harm.
But honestly I would just ask them to begin with and gauge their response before I go searching for his D.C. If you’re not convinced by the response then follow up - and keep digging, for the safety of your D.C.
It could be out of his control over something that happened with his ex. Equally it could turn out that he is not to be trusted around children. It’s strange your DP lived with him for so long, knows he has D.C. but doesn’t have any idea of his story 🤔