I've yet to see a mature adult who willingly sit next to someone else's 4 year old vs. switching seats, but in a day of paid seats, I can't say I'd expect the guy who already paid $88 for an aisle/window seat to move without some form of refund or compensation.
Many years ago, my young daughters were upgraded to domestic F and were assigned window seats (like 2A and 2F) while I was assigned a window seat at the opposite end of the cabin (probably 6F).
The gate agents were unwilling to try to solve the situation but the first class FA made a valiant attempt. When that was unsuccessful, I intervened and simply asked the two Type-As in the aisle seats why they were both so adamant about sitting next to cute five year old girls who weren't their daughters, and within about two seconds, they were tripping over each other to re-seat themselves so that my daughters could sit next to each other.
Others in the cabin, upon learning of the situation, quickly opened up an aisle seat for me across the aisle from my daughters. I was very surprised it came to that - the two Type-As (just had to sit in an aisle and could not occupy a window in first class) were obviously two of the duller knives in the drawer that day. I wasn't worried that they'd be molested - but the sisters had planned on sharing the books and activities they had brought on board, and they tended to want to talk to each other (alot), and could not do that with two pudgy, knit-shirt clad businessmen (with their respective company logos) between them.
On every other flight we've ever been on, passengers up front have always been willing to move to help reunite couples or siblings. I have never refused a request by other first or business class passengers when I've been traveling alone, and frequently volunteer to move when it's apparent to me that a couple is separated and if I move, that resolves the issue.
Fortunately, owing to top-tier status and occasional paid premium tickets, I've never tried to resolve this issue in the back of the bus. Up front, a window is almost as good as an aisle, while in the back of the bus, that isn't always the case.