On several threads on mn if there is the mere mention of grandparents doing some childcare inevitably garners responses of "free childcare" and other indications that parents are holding their parents responsible for childcare.
In our case we were more than willing to pay for childcare 5 days a week.
Pil asked us could they have ds 1 day - they love him dearly and knew as soon as mat leave was over they would not see as much of him. Obviously working full time our weekends together are precious. So we agreed and this has been going on for 8 years.
The advantage is we save a days paid childcare but pil and ds are the real ones benefitting. There is a fantastic relationship between them. If we are off work etc they really miss that time with him. If I so much as offered a Euro they would freak out. I used to drop in meal vouchers every few months but mil really didn't want them. She sees it as precious time with ds (and now she does the same for dh niece).
So aibu to ask that you don't jump on the bandwagon slagging off parents who have "free childcare" and realise that sometimes it is at the grandparents request!