3.5 year olds not fully potty trained?

Hi! I've already posted this, but I'm reposting it with a bit more explanation due to the negative response I received. I should note, I really, really care about the twins that I nanny for (almost four, male and female). Anyways, they're almost four, but not completely potty trained. They are completely healthy physically and mentally and have the bodily control as they have used the toilet before and know when they need to go. However I feel like if they demand a diaper from me especially when they need to pee, that I'm just further enabling them by giving it to them. I honestly don't see a reason to keep them in diapers just because they want to be in diapers, I mean I've seem them with super sore bums from having a full diaper on all day in kindergarten - I don't see how that can be justified but being firm and saying 'no diapers' can't even though it would put an end to needless pain. I should also say, I p no judgement onto their parents for this, I can sympathize that after a long day at work you don't want to have your child screaming and yelling and crying when you could just put them in a diaper and be done with it. Also, I understand that this is a usual phase for lots of children - peeing in a toilet but not going BM, but for almost a year? Isn't that a bit long, and becoming unhealthy? As I said, I really do see it as enabling them by giving into their demands, what if I gave them chocolate, candy etc every time they demanded it? Also, they are fully aware that none of their peers in Kindergarten are still in diapers, and they've started teasing each other about it (ironic because it's both of them) so I feel they've internalized that they probably shouldn't be in diapers at this point but have become fixated on it. SO, my question, now a bit more explained, still stands as: do you think I should, as a caregiver who sees them every day, put my foot down and refuse them diapers? For peeing in? For BM's? ALSO, I have talked to the parents about this and the inconsistency kills me - sometimes it's "yes just put them in diapers" and other times it's "no, stand your ground and don't give them a diaper".