The reason your child wets the bed is not just because there is too much liquid in the bladder. Think of it this way -- even if you drank a gallon of water before bed, you’d wake up to empty your bladder rather than wetting the bed.
Nonetheless, limiting the amount of liquid your child drinks before going to sleep makes common sense because it will delay the filling of the bladder and give your child some extra time before bed-wetting occurs. This added time could give your child more of a chance to wake up before wetting the bed.