OP, you are providing a couple of hundreds of pounds worth of care a day. Financially, you are being exploited.
I know that you love your DGC and want to help but, really, it is perfectly OK to ask for some assistance with the cost of feeding someone else’s children.
Hint that you might not be able to offer the care so the DIL has to face the reality of paying or juggling childcare options. She will soon come to see what a treasure you are and cough up.
Lack of public toilets in towns.