Depending on the age of the child, I may well decide to 'share the middle' seat as the parents, budging my butt and arm over into it and putting the child in between me and the window. That way they can be propped up (as I said age is a factor) and not spit up on / scream in the ear of a stanger. Me + lap child is less than 1.5 seats IMO, thus entirely fair.