My son wet his bed until he was about 8 - he was very embarrassed about it and never went on sleepovers. Eventually we went to the GP who gave him medication which definitely helped, he was quickly dry. Sorry, don't remember what it was.
I think it happened because he was very energetic, burned a lot of energy during the day and slept very deeply. It was actually very difficult to wake him and it seemed pointless to lift him late at night to encourage him to wee when he was really still asleep.
Not an unusual problem, children are all different though, so really the waterproof sheet solution seems the best to me as your GDIS still young.
Last weekend, in Rutland, the first statue in Britain of the late Elizabeth II was unveiled.