And the children DO tidy up after themselves. It’s more like (and yes I can understand I might be being picky) food left on the table from breakfast and lunch.. used glasses left around, their change of clothing on the floor, shoes just dumped where they’re taken off.. I do encourage my children to clean up after themselves.. and I’m not being particular about the toys being out as that’s what they are there for, to be played with
It doesn’t sound like your children tidy up for MIL! Maybe she tells them to put their discarded clothes/shoes/cups/toys away and they claim ‘mummy always does that!’ Or they just say no as she can’t/won’t enforce it like a parent.
My DC have a rule that each toy box is put away before a new one comes out. The toy boxes are out of reach. Why not tell MIL this rule and ask her to make them put toys back in one box before she gets them another? Eg put playmobil away before the Lego comes out, fancy dress box packed up before the farm is out. My 6 year old DD was fully capable of this when she was 5 just reluctant and would play us off against each other eg ‘but daddy said I could leave my craft things out for tomorrow’ (daddy said nothing of the sort!)
Why aren’t all shoes left at the door/entrance hall? Why isn’t 5 year old putting her own laundry in the laundry basket?
As for plates or glasses or food left lying about, if they were MIL’s half finished cups of tea or snacks and it happened daily I’d be mildly annoyed. But it still only takes 5 minutes for you to gather them up.
If she’s leaving food out for the kids are kids not finishing their meals/snacks/drinks
maybe she lets them graze? If she’s cut up fruit or provided things like breadsticks, nuts, crackers and veg slices she could be leaving them out for kids to finish? I’d only be worried if the food could go off each leaving fridge items out.
Or is she cooking them lunch and leaving dirty pots and pans all over the kitchen, dried food on the table etc? That would irritate me. But as she’s providing free childcare I think you could provide some pre-prepared lunches for the kids and her, or things she can quickly heat up? That way any cleaning will be minimal.
Alternatively you can send them to nursery or a childminder against MIL’s wishes?