It amazes me that so many parents traveling with children on planes are so clueless. Like you my first (and only) flew as a babe in arms at 4 months. I asked the pediatrician before I flew and she advised that as well as feeding him through take off and landing to help his ears (since the ear canal is so small) to give him baby Tylenol 20 mins before take-off and 20 mins before landing to help with the ear pain. I am not a big proponent of drugs but the baby Tylenol saved his ears on many, many occasions until he was about three and was able to chew gum. I too have watched children who are obviously in terrible pain with their ears. The parents apologize to me for their child crying but I have extreme sympathy for the kids (albeit I do not like the noise), and inform them what the pediatrician told me 20 years ago - they are amazed they have never heard of that preventative solution.

(The other tidbit of advice was to ensure a blanket was over the bassinet so the baby was breathing filtered air as at four months their immunity is quite low and trying to deal with the recirculated air with all the germs is a lot for them - obviously the bassinet was not totally covered as they still need to breathe and I did not want him to suffocate but he did not get sick after his flight(s) while he still fitted in the bassinet.)

There are number of people who I may prefer were not on my flight, but if I am flying on a commercial aircraft, i.e. not private/corporate jet/NetJet, and United sold the pax a ticket then I do not have any right to complain about who they are. If you believe you should not be on a flight with infants/children, then you need to find an alternative that meets your requirements. I would prefer not to be on flights with animals in the cabin since generally the animal dander brings on allergies for me but while the airline allows them I can only ask to be moved to ensure I am not near them - fortunately many of my flights are to destinations with strict quarantine rules.