Potty Training Question

[busydaycarelady]

I have a little one who is well over 3 in my care. The kid has finally decided to pee in the potty and now does so all the time at my house. Refuses to poop in the potty though. Will poop in their undies at LEAST once a day, generally 3 times. This child will even sit on the toilet (when I KNOW the kid has to poop) for 20 minutes and then the INSTANT it gets off the toilet, POOPS in the undies!!!!! As in, standing beside the toilet pooping. ARGH!!!!!!!!!!!!! The parents solution is to put the kid in a diaper at home all the time(the kids will still sometimes use the potty at home, but not always because, well, it has a diaper on so why not use that?).......which is thwarting the progress we WERE making here. I suggested plastic pants over top of undies and when this kid poops, leave it for 5 minutes before changing the undies, just so it sinks in that this is NOT comfortable. Parents were okay with that idea, but, didn't want to have to wash undies AND plastic pants after, so at home diapers it is. The kid literally gets into it's home and mom and dad put a diaper on. So frustrating!!!!!!! Any suggestions?