It is USB charger

Yes you can hook up two of these to the same device however you can only use one at a time. You can use two at a time by connecting one of them to the Bluetooth and use the connecting cable provided that connects both sets of headphones together to hear the same thing.

I believe they need to be charged ahead of time. But they don't have to be plugged in to the power source to use audio jack.

Yes. It come auxiliary cable which can be used on the plane entertainment system

YES! Actually, I am not sure it’d fit a 3yr old well? But I can not say for sure as I do not have a 3yo to try it on. Fits my 8yo well w/room to grow