I have found that for some odd reason whenever my daughter and I fly together, our seat assignments (usually in paid F) get separated. She has been flying in her own seat since she was 6 weeks old. She is now 5 years old. I can't remember the last flight I was on with her where I DID NOT have to ask someone to switch.
Your situation would be one where I'd be most willing to move. Main caveat is will not move to a middle willingly, to a worse middle if I'm stuck in one to start with, or to any "torture seating" rows (seat pitch designed with small 11 year-olds in mind) that I know exist on some planes. Other than that, willing to switch aisle or window or vice versa if needed to accomodate a young child if travelling solo. I prefer one or the other in certain circumstances (over the Arctic and other scenic terrain I like windows, otherwise prefer aisles) but am not locked in.
Other situations it really depends. I've moved to allow a companion to sit beside a very elderly and frail relative (why the airline separated them in the first place I'm not sure) who was gracious, but not moved to accomodate a married couple who were twits. Courtesy is key with me - I'm a Canuck after all
. Rudeness/overentitlement will get a firm no and the only one that will get me to move at that point (unwillingly) is an FA.