I just think there's a difference between "house rules" and rules that parents have for their children. A lot has changed since we had our kids and there are new discoveries all the time. Today, it seems, many doctors recommend against giving juice as early as we used to, and some parents avoid it even with a 3 or 4-year-old. "Granny's house/granny's rules" doesn't really apply here, IMHO. If the parents say "no juice," then it's "no juice" even if "Granny" doesn't agree.
Also, some parents are skittish about anyone driving baby but themselves, car seats or not. One of my DDs was like that for the first year of her first child's life. We respected that and didn't drive baby anywhere whenever we watched her during that first year.
But, IMO, there's another side to this coin. If a GP really can't deal with the any of the parents' rules, they need to let the parents know. Then the parents can decide if they want to relax the rule or make other babysitting arrangements.