Disposable diapers don't really expire according to the diaper manufacturers.
However overtime the plastic beneath the cloth outer covering of the disposable diaper could deteriorate and fall apart.
If you've ever touched a plastic bag after it's gotten older you'll sometimes notice that it might fall apart in your hands.
That's about all that could happen to the expired diapers.
As long as the diapers have been sealed in the package and not let out in the open in the sun then the disposable diapers should be just fine to use even 10 years or so later.
Disposable diapers take a very long time to really decompose so as long as the diapers are stored properly then they should be fine to still use.