You son of a bitch, you think your happiness and comfort takes precedence over what some spoiled little kid wants???
Seriously though, I wouldn't have done it either. If just smile and say, "no thanks, I'm going to try to eep and sitting in the mide between a kid/mom and the dad on the other side dosent sound great to me.
Now if the kid was on the other side of the isle and they wanted to trade because the kid wanted to sit with the mom, or husband/wife, bf/gf....I may do it.
But bending over for some little kid...nope. It's the parents job to entertain the kid and nobody else's.