I was going to suggest a materity pad stuck up the top of the nappy near the belly button, but cailin beat me too it -lolol. It at least soaks up the wee from their body.

Otherwise I use to just change my boy just before we went to bed, even if he had only wet a little. A new nappy was then able to last him the rest of the night. He got use to being changed and would roll over and go straight back to sleep
hope this helps