Seat swaps, flying with kids, etc..
My advice would be to trust your fellow passengers to handle it, and to help you out. Most people are pretty awesome, and can be highly self-directed / helpful if given the opportunity.
I fly almost weekly with my Daughter (7) and we are often seated miles apart. 90% of the time, as soon as I direct the kid into her seat and tell her to "be good, mommy is just 25 rows away" her seatmates are falling all over themselves to offer me a seat and trade places. I try to thank them profusely, give them a hug, and buy them a drink for their kindness. long haul, short haul, 16 hour flights to NZ / the Middle East, people are always wonderful about swapping seats.
On the rare occasion when they don't offer to swap seats, I'll tell them where I'm located, offer to swap if she becomes a problem, hand the kid a stack of books, and enjoy a few hours of child-free bliss. Often her seatmates are empty nesters or travelling dads / moms who seem to relish the opportunity to do the grandparent / parent thing for a couple of hours.
In two + years of weekly flights, we've yet to have a negative experience with pax and we've never once had to ask the crew to intervene on our behalf.