3 1/2 and 4 1/2 year olds still in diapers?

Well, as it isn't your child it makes the issue hard because anything you do will simply be undone when at home. But, if you're watching them make them sit in the filth and make a big show of how gross it is. I know it sounds crazy but make him feel ashamed of what he's doing, when you go to the bathroom come out and be all proud and smiling and say. "I just went pee on the toilet, I'm such a cool person." If he knows other kids around his age or younger who are potty trained make a point to tell him that and say "They're so cool,", and tell him how cool he would be if he potty trained. With the girl I'd take a slightly different approach, with just showing her and rewarding her with maybe a treat and lots of praise. I think also if you get the girl potty trained it will help the boy because if he's little sister is doing it and he's not then it might be upsetting. I'd also make a point of telling there parents that next year or the year after the boy is going to be unable to go to school or any sort of preschool