Assalam O Alaikum,
Yes, few children have this habit, do not worry, it is not a serious issue. I have seen children at the age of 10 to 12 who urinate in the bed during night sleep.
Try to give hot natured foods in small amount frequently, egg, sesame will be perfect.
Paste of Qust powder (half teaspoon) with a teaspoon of Honey will also give enough heat in the body & your child will be OK.