If they agreed to go then what they did is absolutely nasty. To let you down last minute with no genuine reason unbelievable, how anyone could do that to their parents is beyond me.If this was created by his wife he had choices, he could have gone with the children without her. He almost certainly should pay for any expenses incurred on the family’s behalf, he should offer without being asked.
I can’t imagine my son or daughter treating me like this, if they did I wouldn’t say anything and wait for the it reaction. Treat your grandchildren the same as always, if this silence goes on for any length of time start sending postcards and letters (dependant on age) to the children.Tell them how much you love them etc etc.
You don’t know your sons relationship could be breaking down.I am sure you know but never make any comment to your son about his wife or offer advice.
I have a know it all DIL she did visit us some years ago and went out every Sunday with us for a meal. Our situation now is our son and the GC come for the day Saturday and Sunday all day and we go for a meal Sunday together without her, it makes for a lovely weekend and we all have lots of fun.