Doodledog
Is your MIL like that about other things, or is it just over your son? My mum really wants to be thanked for everything, and of course we (my siblings and I) do that, but it’s never enough. If she buys an item of clothing as a present (for instance) I would say thank you at the time, but she would ask a while later if it fits nicely, or if it washed well, to get more thanks. Anything she does is spoilt by the constant reminders of how good to us she is, and we do thank her when she does them. Childcare was never an issue for us as my mum rarely looked after mine (she didn’t approve of working mothers).
I don’t know if it’s a generation thing, as there are a lot of posts on here about people wanting more thanks for presents, and comments that ‘we have been very generous with money’ before pointing out something that posters wish their children had done. Gifts of time or money seem very loaded sometimes, and whereas it is perfectly reasonable to expect a thank you, they can be spoilt by constant reminders of indebtedness.
Having said that, it’s annoying, but in the end, it’s just words, and if she’s made happy by your thanks, why not just thank her? Why not start a conversation (very chatty - not A Conversation) over a cup of tea and say that you’ve noticed that she gets tired after looking after your son, and that you don’t want her to feel under any obligation- stress that he loves her, but make it clear that the outings should be fun for both of them, and if they are tiring for her it’s fine if she rations them a bit?
You sound like you are a caring DIL, so you’ll probably find the right way to phrase it without hurting her. Good luck.
She is like this with many things. If she gives a gift, and I say “thank you! this is so generous,” she will come back with, “Well, yes. I’ve been VERY generous with you. I always am.” I just don’t know how to react to that. Im not sure how much more gratitude I can express. When we try to bring her gifts or food to show we are thinking of her, she often says thank you but pushes us to bring it back with us (wine, flowers, food, etc - and no, she doesn’t have any issues with alcohol, so it isn’t that). I’m just at a loss.