Hi - We've had one for a couple years and what a lifesaver!! Battery life depends on the brand - so why don't you charge it up and then watch a movie at home and see how long it lasts! I bought an extra battery from the manufacturer for mine, because we were doing cross-country trips via plane, and the 2 1/2 hour movie wasn't going to cut it!
Southwest doesn't have plug-ins for electronic equipment. Some other airlines do, but not every seat - you can check
www.seatguru.com ---- but you would have to know the type of aircraft you are flying (but again - not with southwest - they're budget - which is why I fly them all of the time, but you don't get the extras like plug-ins or assigned seating!). There are plugs at airports if you want to burn up some waiting time with a movie without eating up your battery power.
One other thing - bring earphones! You can get a splitter at radio shack (cheap) so that both of your kids can listen at the same time thru their own earphones (it splits the signal from the machine to 2 earphone jacks). My son likes the small earphones that fit in his ear with a band over the top of his head (ear buds don't work because his 4 yo ears are too small to keep them in without the headband) - but you may already have some the kids like.
Earphones not only make for friendlier fellow-airplane passengers (most of whom do NOT want to listen to Dora the Explorer ), but they are also great for the car rides - because then you and DH can have your own music, plus adult conversations!!!

Jill