My mum has looked after DS once, my DMIL once. Geographically, it is harder for my DM to help, but we often go and stay with her for 4/5 days at a time (at their request). There is never, ever an offer of looking after DS so we could pop out, even for an hour. My DMIL lives much closer, but again, only the once.
They both, on a weekly basis, look after their other grandchildren.