In my own experience - the airlines I flew when my daughter was young enough to be a "lap child" went out of their way to ensure that, apart from take off and landing when she HAD to be on my lap, there was an empty seat next to me and I was actively encouraged to use it for her in flight by the cabin crew.

The only time there was a pretty full flight, they actually apologised to me at check in that they couldn't give me a seat for her. However - when the middle seat passenger arrived on board and saw us seated by the window , he threw a complete tantrum, refused to sit down and demanded another seat well away from us. I have no idea where he ended up - but I did in fact end up with a free space to put her beside me while I enjoyed my drink and meal.

I can also honestly say that not one of the aisle seat passengers I travelled with on those flights ever raised any objection to the baby using the empty seat. Maybe it was because the crew made it clear that I was to use it and came around asking if I needed bottles warmed etc, or maybe it was because as a reasonable human being, they didn't expect me to sit for several hours with a child on my knee when the seat was available and a child had more right to a bit of comfort than their books, bags or newspapers did.

I flew each flight in the knowledge that I COULD end up with her on my lap for the duration, unless i bought her a seat - but as I said - it was the airline and aircrew who offered the space - I never asked for it or expected it as my right, and I don't think that MOST parents travelling with "lap children" do either. If there is an empty seat - fine - if there isn't that's fine too.