I'm going to be the bad guy here and admit we did it. On our flight to my MILs, my LO did fine. It hurt her ears for a little bit and she fussed, but other then that it was no big deal. We didn't give her anything because we didn't need to.
However, coming back was a different story. She was terrified, crying, shaking, clutching my clothes and burying her head in my chest. We had anticipated this because she has had problems with anxiety and being overstimulated in the past, and its usually at its worst after being around strangers. The only way to resolve it was to remove her from the situation. Obviously it's not an option on a plane. Our doctor was fine with us giving it to her for anxiety and figured the dose for us.
I don't feel bad about it. Doctors and hospitals use Benadryl for anxiety. It's only sold to the public as allergy medication, but I don't see a problem with using it for anxiety on a doctor's order. People take anxiety medication for flying every day. Just like I have to determine if she needs Tylenol for teething, I have to decide if she needs Benadryl for anxiety. It wasn't for the other passengers' sake. It was to calm her down. She screamed for the first twenty minutes of the flight anyway. That's the one and only time we've used it.