FWIW most airlines require the use of headphones for electronic equpment - so IMHO if you are prepared and prepare your child you can avoid a potential problem. My kids knew that on the airplane the movie had no sound unless they wore headphones - their choice.
Walmart and Target have some childrens headphones with characters or without in smaller sizes for comfort.
From american airlines website - most other airlines have the same/similar rules -
"Equipment/Personal Devices:
You may use audio and video devices only with a headset. Use of still and video cameras, film or digital, is permitted only for recording personal events. Photography or video recording of airline personnel, equipment, or procedures is strictly prohibited.*
Note: Hearing aids, cochlear implants, heart pacemakers, or approved medical devices such as ventilators and respirators are accepted during all phases of flight.
*The electronic device policy may vary on American Eagle and AmericanConnection. Please see a flight attendant for specifics."
Many of us have kids and have been there done that so the advice is not sugar coated. I don't mind kids on flights at all - I love to hear the babble, the mom telling stories and playing peek a boo! Dora the explora, not so much anymore

Everyone knows when parents are trying and when they are tuned out - when they are anxious about little Bobby kicking the seat and when they don't care or think its funny....
It isn't easy travling these days but to be prepared definatley helps as the OP is trying to be. Bring a big bag of tricks, snacks and small quiet toys - remember that you are the best toy of all - finger games, peek a boo etc.
Once my dd at 15 mos played with a roll of scotch tape for an entire 4 hour flight so you just never know. Stickers and a removable book were a big hit at that age as were pipe cleaners.
I assume you are refering to my post. Which was not only about this thread. Its an overwhelming attitude on the dis regarding parents and children. Which surprises me given the content and purpose of these boards. But that's not the point.
In a perfect world, these inconsiderate parents would be able to perfectly control the behavior of their toddler and this toddler would either use the headphones or not get to watch and would then sit quietly for the rest of the flight. In my world things rarely ever work that way with toddlers.
We're not talking about a 5 year old blaring Mario bros on a nintendo ds on its highest level here. We're talking about a small child listening to a movie or show on a quiet setting. You're on a loud plane, its not like you're in a meditation garden. I do not see this as inconsiderate.
Like I said, its not just this thread. Its an overall vibe on this dis. about who should be considerate of others and who shouldn't.