I have suggested to my DD ( in jest) that she could employ me as their nanny or cleaner, then I’d be allowed in their house.
Not all nannies or cleaners are young
nannies and cleanera who are in shielding categories are not returning to work.
If you have your GC who usually go to nursery/childminders a day a week, or for a sleepover at weekends, thats socialising not formal childcare.
However if you do the daily childcare to cover your ACs whole working week. Same as a professional would. And are young (I know grandparents in their 30s and 40s and 50s) and arent shielding or working yourself and can cover the full weekly requirements of childcare. That makes you their childminder/nanny. And you can go back to work.
A day a week... you were never their childminder/nanny. That waa for you to socialise with your GC.