Howdy! So, the
DAS covers the holder (in this case, your nephew) and 5 additional guests. So it seems like, if it just you 4, that you would all be able to stay together. As far as the advanced bookings go, no. You cannot change them once you arrive, and if you do not want to do them, you will forfeit them.
Nephew also HAS to ride any ride you get a return time for. So if you got a time for It's A Small World the day of (not in advance), but when it is time to go, he decides he does not want to ride, no one can ride on his DAS return time.