There are many theories about what might cause children to wet the bed at night, or in this case, to wake up frequently to urinate. One of the most accepted theories is that the body creates a hormone called vassopressin. At night the body should create a surge of vassopressin that causes the body to condense urine (that's why urine is usually a darker shade of yellow in the morning). Some bodies don't produce that extra surge at night, and so the body doesn't condense the urine and thus you have to wake up more often to go to the bathroom during the night.   Some children also might have a smaller bladder that hasn't "caught up" to their body yet.   Good luck!