A few years back flying Turkish airlines, scheduled on an A321 and had seats on the back row, whilst boarding in Istanbul when scanning was advised that aircraft changed to A318 (IIRC) and reseated. 4 completely different seats for my wife and I plus 2 and 4 year old children. I thanked them for arranging for other passengers to babysit my kids, and hoped my kids wouldn't get airsick on those fellow pax... They coordinated us all together pretty quickly. :-)