What is the amount of water needed to purify the clothes of a child who urinates at night, and is it sufficient to immerse them in water?

There is no specific amount of water required to remove impurities; rather, it is sufficient to use whatever amount removes the impurity itself and its trace. For major impurities like urine, it is enough to pour water over it repeatedly until it is gone. If it does not disappear without scrubbing, then scrubbing becomes necessary. Submerging clothes in a basin of water is sufficient to purify them if the impurity itself is removed, provided the water is not so little that it becomes impure upon contact with the impurity. Washing clothes daily does not negate the need to wash them if they have been soiled with urine.