Look, I understand you're pissed off with him. I have a tosser XH of my own!
But you need to move on from the stroppy responses about falling under a bus! If that happens, your grandparent contacts will travel to your home until he can be contacted / get home - and social services will step in until they arrive.
You need to focus on what you CAN change. Did the kids have a call with him every day when he was away when you were still married? Is it realistic to do that?
I'm sorry it is unsettling for them. But I think you should work out what they want and what is realistic. Then mail that to him. "the kids are unsettled, they would like a call on x days - what do you want to do?"
Then let him contact them directly, they're old enough. If they don't hear from him, be honest, don't cover for him. Let him tell them why he didn't call.
FWIW, my daughter (she's primary) and I don't speak on non contact days. I've told her when she's old enough for a phone, she can call whichever parent she wants, whenever.
If your kids need/want more contact with him than he is stepping up for, because they're are feeling worried and insecure, then I'm afraid talking them through that will fall to you.
But you won't solve their anxiety just by you having his diary.