Case 1 After she left with the kids, a few hours later, she text me telling me about how my oldest S laid on W new mattress and said "wow, I'd rather sleep on the floor Mom".
I just made small talk back.
Case 2 Later in the night, she messaged me again about some small details with the kids and school, some logistical stuff. She ended it with saying that she would go to the store to get some materials for our son's school work.
I dint respond at all after she told me how she was going to handle the issue.
For me, I think I would have done this in reverse. In Case 1, I dont see any reason to respond to that. If you want to say something like "haha" or "lol" or whatever youd typically respond, then fine. I certainly dont see any reason to 'make small talk'.
On the other hand, Case 2 is something that directly impacts you and your son. I assume by her doing that, it means you dont need to go to the store. Or it's something your son will have for school to improve his work or whatever. In this case, I think it is good to reply to acknowledge her efforts for S.