Disposable diapers should never be reused unless it's never been peed or pooped in and you just removed the disposable diaper from your toddler or baby to give them a bath.
I've babysat my nephew before and his diaper was still completely clean when I decided to give him a bath and so I reused the disposable diaper that was still clean and put it back on him.
However if it's been peed in even a little bit you should throw the diaper out and put a fresh diaper on your child.
But you don't have to change the diaper if it's just a little wet since disposable diapers are designed to absorb a lot of pee before it needs changed and the child will not even feel wet.