It has been so interesting to read everyone's views on this - I really have appreciated all the perspectives.
We almost ended up being me, DS (4), my dad, and his girlfriend in 2020...but since that didn't happen, and by the time it does happen, my son will be at least 8, so we'll definitely have 2 rooms.
DS and I are going with my mom in January (the 'surprise' that may or may not stay a surprise) - we're in one room, which is no issue, but the bed situation is on my mind. My mom really needs to the queen, so either I sleep with her or on the couch...DS will want the bunk (and I am assuming there will be one on the Wish, although clearly not true in every stateroom on Deck 2 as we learned on the Wonder).
I like the idea of rotating - although it will require good comms with the steward.
I would have slept with one grandmother when I was younger but not the other - but only because she snored very loudly and I was always afraid when she stopped snoring that she might have died. And of course I couldn't tell anyone that in my 8 year old mind... So, PSA to all, while your kid might love their grandparent very much, they might not want to sleep with them.