My seat was reallocated once, so I was next to an unaccompanied minor, instead of the male passenger who had booked the seat I ended up in.
This reorganizing was done before we boarded the flight, so I got no choice in the matter, I was just informed of the change by the stewardess when I boarded.
Never mind the fact I'm 6ft 2 with very long legs, and had specifically booked a seat with more legroom, but was now stuck in a more cramped seat. When I protested, I was told they wouldn't reorganize again, now the flight was boarded.
So not only did I have to spend a ten hour flight entertaining someone else's kid (this was before tablets and whatnot), I was bloody uncomfortable the whole flight.
It wasn't being seated next to the kid that bothered me though, it was more the fact they refused to change the seats again, so I didn't get the extra legroom I'd booked (and needed).