Jayna has achieved nighttime dryness! *applause*
Joash could do it at 4 years old without much fanfare. I simply noticed that he had not been wetting his bedtime diaper for quite some time and asked him if he wanted to wear his underpants to sleep instead. Apart from an accident or two, his transition was almost seamless.
Jayna, however, needed more motivation. I used the sticker system: 10 consecutive dry diapers in order to proceed to wearing panties. Any leak meant that she had to start all over. It took a few months, but when she finally reached the 10th stickers, to stay dry for another 10 consecutive nights to earn the new set of stickers was a piece of cake for her. (Note the 20 stickers on the sticker board to chart her progress.)
At 4 years 4 months old, Jayna can now say goodbye to diapers for good.