Good for you, Nelly! And DH!
Not sure what there is to discuss with DS and DIL though. DS has already said what's needed. And, right now, if you voice your concerns, it might not go over well. If you apologize for past errors, IMO, you need to keep it simple. No ifs, ands or buts. Otherwise, they might see it more as an attempt to justify yourself and they won't take it seriously.
"We never know when we will see them and even if we make plans they quite often cancel on the day for various reasons."
Meant to say earlier, this stinks! Seems very unfair to me. Ugh! I totally sympathize.
True, with a little one, there can be unexpected issues. Or this could be due to the lingering tensions between you and the parents (sigh). This, too, may change as things get better. If not, then maybe you can address it if you'd like. For now, I hope you and DH always have a backup plan.
Best of luck!