Disney's reservation system is rather odd. In the past, I tried to change my ADRs from 8 people to 3 (due to a death in the family which caused part of our party to cancel their reservation). I couldn't do this. In order to change the reservation, Disney would cancel my current reservation and then had no space for the smaller party.
At the time, I was absolutely sick with this situation and the guest services person was mean and nasty about the entire situation. Most people at Disney are wonderful but this was awful. I went to the concierge desk at OKW almost in tears. The concierge recommended that I keep my reservations and explain my situation to the hosts when I checked in for my meal. This woman was wonderful and so sweet. I remember being amazed at her positive attitude. At the time, she had been watching her son struggle with brain cancer.
So, I took her recommendation. Every Disney host/hostess was understanding. I'm sure that you can find someway to accommodate your child. I'd keep your ADR's and have a respectful and flexible attitude. I'm sure that you'll have a great time and find people that will try and make your day truly magical.