We have a three year old and, for some time now, have realized that young kids need something somewhat larger to really engage with the audio of in flight movies. Perhaps older kids might be fine with
kids earbuds but here are some
young kids headphones that aren't expensive and will certainly do the trick. The top pick definitely has worked wonders with ours.
One thing I'd recommend is to make sure whatever you get isn't Bluetooth only. By having a cable the kids will be "forced" to remain seated in order to hear what he's watching, which is a nice thing to know IMO.
At any rate, no travel for now...