A: Nighttime dryness requires the ability to have a bladder that's large enough to hold urine all night long and/or the ability for the brain to "wake up" and feel the urge to potty in the middle of the night. 90% of children achieve this milestone by 7-years-old. Having a child wear underwear at night when there are frequent accidents only leads to a soggy mattress, so consider putting your child in diapers at night. Then limit beverage intake after dinner and take your child to the bathroom right before bed. Once she wakes up dry consistently for four weeks, take away the diaper as a safety net.