"Being with grandchildren sounds lovely (esp to tell her friends about it) but at no point was she ever really 'with' them."
Yes, my MiL is the same
We went away with her a few years ago - she went on and on and on and on about going away for a family holiday, she wanted to spend time with her grandchildren, we had moved away and it "broke her heart" that she didn't get to see them, it would be lovely, she'd babysit, we could have lie ins as she'd get up with them and take them for early morning walks, etc, etc. We'd already had a holiday and couldn't really afford another one but she insisted she would pay.
However, as she was paying it had to be where she wanted, so ended up cramped into a cottage which really wasn't family friendly in the middle of nowhere. Lovely for sitting on the terrace with a glass of wine and looking at the scenery, but bloody awful with bored 5 and 9 year olds.
When it came to going out she refused to do anything that the children might like and just wanted to go off alone with her husband for the day. We were expected to meet up in the evenings to go out for meals to the restaurants she wanted, which were never somewhere you could kick back and relax and let the kids knock themselves out playing with other kids in the pub garden.
Wanting to "spend time with my grandchildren" seemed to translate as lining them up for the odd photo she could show her friends.
We left them to it in the end and did our own stuff and had an OK time, but we'd never had gone if we had known it was going to be like that. I don't expect them to look after my kids, but they refused to make any concessions that may have made our lives easier (eg, relaxed pub rather than formal restaurant for dinner), refused to spend any actual time with them and just made a week a whole lot harder than it would have been if we'd stayed at home.