R.R.
My daughter slept in pull-ups up until she was 9 yrs old. She is a very heavy sleeper. Also it runs in the family fathers side. I did take her to to pediatrician twice year. I was told not to worry.
She is 11 now and still a heavy sleeper- but completely out of pull ups at night. My night time potty training with her at 3/4/5/6 yrs age didn't work.
Hope this helps. It might take a little bit longer if they are heavy sleepers and thier internal clock has to kick in to wake up.