Bed wetting is a very common thing in children and can last up until the child turns 15.
Bed wetting is usually caused by the child's bladder not growing as fast as the rest of the child's body and the result is that the child produces too much urine at night overflowing the bladder which causes the bed wetting.
Bed wetting can also be caused by stress as well.
If your child wets the bed you should talk to the child's pediatrician as they can prescribe some medication to help with your child's bed wetting.