DesertRose,
I sit for our grandson from our oldest AS every Tues. Thurs. and Sat. (and I love every minute)
The situation between my son and his ex GF is very strained, she even filed domestic abuse charges against him. (they were recently dropped)
But...My son KNOWS his son is here. If he wishes to visit, and he's sober, that's okay with me.
IF you think your son is using, then IMO, kids come first, no contact.
Do YOU want the kids to come, or are you feeling as a grandparent, a responsibilty to have them there?
OR...if your son wants to see thie kids, make a prearranged mtg. at McDonalds, or somewhere.
Hugs..................