If it's a newer plane they will have a USB right in the middle for you can charge your phone or two devices so all you have to do is go to the store and look for a phone charger that will fit your switch and you can charge it cuz I do it on my wife's cell phone charger
Make sure the power bank you purchase charges at 5 volts (or watts idk) the same as the wall charger. The switch power is just over 4000 mHz and so I have one thats 10 000 so u can charge it twice. Puchase a USB C usb cable, and then u can charge it with the port. U can charge it on the plane but I'd make sure it charges at the right speed first. (5 units at a time wh ich is quite high)
For those who said to use the USB plugs on the flights, no, they wont charge the switch while playing. All you need to do is to use a power bank, but make sure you HIDE it while charging, because no flights will allow you to use a power bank while its flying. ..