I’m not sure whether giving grandparents, who are providing free ad hoc childcare, a strict list of your childcare rules is necessary. You seem to suggest that their contact time has been reduced as a result of them feeding too much cake, varying your dictated nap times, nappy changing etc. Of course, it is essential that GPs keep your children safe, but I wonder whether their care choices should, to some degree, be their choice - provided of course they are safe, albeit not necessarily ‘healthy’. Even more so if it is for limited periods of time.
I am not a grandparent. I am a mother who had similar gripes about my mother’s care of my children. I objected to cake at breakfast, endless jam tarts, overlong naps, late nights when they slept over, ‘heavy’ nappies when I collected etc. In hindsight, I now recognise that my objections were probably, in part, based upon: feelings of jealousy, a lack of confidence in my role and a desire to reinforce that they were MY children, that I had control over, and not my mothers! I was trying to stamp my authority, but found reasons to justify my objections that I truly believed in at the time. Your objections do sound reasonable, but I am just offering an alternative viewpoint.
As it turned out, my children remarkably suffered no ill effects from the skip loads of cake for those short years, but had a magical relationship with my mum. I benefitted greatly from her spasmodic free childcare and have appreciated this more now, as I realise not everyone has this support.
I have no opinion on the caffeine issue, but perhaps your doctor could advise and you can then decide whether this is something you really want to push. You may also find that when your children start school, you don’t always agree with the choices made at their setting in loco parentis.