Don’t worry about it, it’s really really simple.
You can pretty much guarantee that your [kid’s] desire not to sit next to a stranger is completely dwarfed by that stranger’s desire not to sit next to your kid.
They’ll be up outta that seat quicker that you can say “Would you mind looking after…”
If for some reason they are slow on the update then simply delve into the seat back pockets, remove two sick bags, give one to the kid and then the other to the stranger “Just in case he fills that one up”
job done
I’ve done it on planes and transfer coaches. Never fails.