Well I hope while we are chatting, you are slipping into the tub.
If not, don't tell me.
Now, he was always a likely candidate for this. Deadbeats are just that. But they do work. They do work for cash so avoid support and taxes. They are enabled by people with no soul. It is sad.
You should decide how much pain and suffering you intend to express to his family. They should know the end effects of how they raised him. They should feel some of the guilt he will not. If they offer any support, take it, it will make everyone but him feel better. He does not know how.
Find out if his company would extend you coverage under Cobra for a limited time. Yes, it will cost money you don't have. Let his family know how much it would cost for your kids to have health care ... "until he gets back on his feet" to do his part.
You won't get anything you don't ask for.
As for LSS, he IS on his way to hard times (in or out of the system). He can blame anyone he wants for his drug problem. But if he has a problem, or develops one, it is his to own. He will need cash and will self medicate his problems. He will probably only get worse from here. Be ready to protect yourself from unhealthy contact. Document all contact and be prepared for a RO at any time.
Get a dry pillow and get some rest tonight. Hold your babies tight and know you are doing all you can to make their world the best place possible.