Chipping in - that's exactly what we did, BUT If I were sat infront of someone with a child on their lap (when we took our 10 month old on his first long haul flight while he DID have his own seat, he could not use it the whole time, he would have injured himself) I would have some consideration for that child. Particualrly if the parent was already doing as much as they could and the child was not crying due to take off / langing, was provided with activity to prevent an unpleasnatly noisy journey etc.
MUCH more comfortable for me, in my opinion, to keep my seat upright, at least MOPST of the time, than allow the child to become upset and NOISY becasue they are squashed and can't cope with the discomfort.
Yes seats are designed to recline, and you have the right to recline your seat, but in civilised society, I like to think that we can compromise and consider the knock on effect of asserting your rights over the rights of all others, if EVERYONE did that, then MOST things would be quite horrible.
If I ahve paid for a ticket on the bus and get a seat, I do not feel that my right to use that seat is greater than the community benefit of my giving up that seat to an elderly, disabled, pregnant, or child carrying person.