well the thing is, if you let them out too much during the night they get the impression that they dont have to hold it. so if you only let them out once in the middle of the night, that teaches them that they need to hold it as long as they can, or they have to sleep in their pee. 7 weeks is pretty young, so if you think that they cant hold it half way through the night, you can let them out a second time.

btw, what breed are your new puppies?