Normally on all ships you get there early. There will be special areas for those with wheelchairs. I have been to quite a few shows and usually there are plenty of seats available, but there might be the odd exception. Most shows are twice nightly timed to suit first and second sittings in the main diningroom.
When you get to the theatre, you will no doubt find the front couple of rows full of children "saving" seats for the rest of their family, who will turn up as and when they feel like it, including AFTER the show has started. From what I have heard, the Venturas theatre is a bit too small for the number of people wanting to use it, so I would advise you to arrive nice and early if you want a decent seat. If you want preferential treatment for having enough money to book a Suite, then I think you are on the wrong ship! Try Cunard or Celebrity. Happy cruising.
We have just got back from the ventura and what burrows said is not true about the front seats being occupied by children.Anyway you cannot book seats and there are no preferences.They put on three shows a night and the early show which is about 7 or 7.15 is your best bet.The middle show (about 9.15) is always packed so you should arrive about 30 minutes early.The late show is normally 10.45 and again is quite packed so give yourself 30 minutes again .
Your cruise must have been full of new cruisers who hadn't yet tought there kids this little trick then? They seemed to have it pretty off pat every night on the "Family Friendly" P&O cruises that I have sailed on. Even that bit of encouraging news is still not enough to get me onto the Ventura I'm afraid. Happy cruising.