Kudos to you for taking on this responsibility for the sake of the kids. It's terrible the parents are being so flaky.
I think in those shoes I'd talk to social workers, lawyers, etc who are helping you with the guardianships. I'm sure they see that sort of paperwork / situation all the time and perhaps can point you to online support systems or real life support systems.
For instance I did a seach for "Grandparents raising grandchildren" and came up with links like
I'd have to sit back an assess how much/little to let the bio parents around the kids. If total flakes, the kids don't need to be around that.
Hang in there. It's a lot of hard decisions but I'm sure you will do your best by your grandkids. Remember you don't have to be everything for everybody, put your OWN oxygen mask on first. Even if it means asking son to move out so YOUR home life is more stable for the babies.