We went when my son was 10 months old too, but this was back in 2010. We also had my in-laws along on that trip as they flew in a day later. This was one of the rare trips that we drove so that we'd have everything my wife wanted to bring. My memories are sketchy as it has been a decade, but the one thing I remember was a stroller.
Practically every photo of my son was in our orange City Mini stroller from back then. This visit we stayed off-campus and we had to rely on him stroller napping to get through the days. As I recall stroller napping did not always happen successfully, so we did have to give up a few times and go back to the off site resort. So, I guess my biggest advice is to not have the two expect too much from the 10 month old and always be prepared to retreat. Once he got an early afternoon nap in, he usually was good to go well into the evening, but our nights were always shortened by having him around. We'd have dinner off resort and basically call it quits, pretty early for a Disney Day. My wife always wanted him to remain on his sleeping schedules.
My mother in law was great and would stay with him often outside rides and queues. She's not a big theme park person and enjoyed watching people more. My wife and I stood in lines and rode rides with my FIL and my 3 year old daughter at the time. I do remember she watched him once with my daughter back at the room and let us go out for a night to Epcot. It was a 10 day trip and that happened once and we were eternally grateful for it.