Should I ask the parents to stop putting her in diapers?

I have been a nanny full-time for this family with two children (3 & 4) for the past year now. When I first started working for them I made it my mission to get these children potty trained. The oldest, a boy, was the first to get potty trained and I received a bonus for successfully getting him trained. The youngest, a girl, would be fully potty trained by now if their parents would stop buying them diapers and make her wear big girl undies full time. When I first started working here they put the boy in diapers all day long and as soon as I would get to work he would poop in his pants even though he was 3 and a half at the time and knew that poop and pee was supposed to go in the potty. So I finally convinced the parents to stop putting the boy in diapers during the day and now they just put him in diapers at night and he is doing great pooping and peeing in the potty. The little girl however is in diapers every day without fail when I arrive to work and if I don't set a potty-timer for every 30 minutes then without a doubt she would go in her diaper. She still does not poop on the potty, most of the time she will run into her closet and hide and poop in her diaper right before the potty timer goes off. She loves wearing big girl underwear and I try to always put her in them but then their mom will get home and get upset if she's in them if they're going to go to the park or something. I just don't understand because it's like their parents are regressing them every time they put them back in diapers and with diapers being so expensive I can't imagine why they aren't focusing more on getting the girl completely potty trained. I'm with them 30-40 hours a week every week so while I'm here I make a huge effort to ensure they're going on the potty every 30 minutes and I really try to watch out for poops because I hate cleaning it up but in my opinion a 3.5 year old girl and 4.5 year old boy really do not need to be in diapers at all and I know these children well enough to know they would do great if they would just be in underwear all of the time. It's like the parents don't want to put any effort forth towards training them though and must see it as my responsibility. They don't even discipline them at all, the children are angels all day long while I'm here and then as soon as their mom gets home they start acting like wild banshees and throwing extreme temper tantrums because the mom just gives into them and coddles them the whole time. The whole situation is driving me crazy, I feel like I make such great progress with them while I'm here and then I go home and their mom lets them do whatever they want so the next day when I get to work I have to start all over again yunno?