I'd go for taking stuff to keep the two children occupied, and then throw in a couple of magazines from airport to be honest. The youngest may sleep a bit of flight. 5yo possible, maybe? But probably not for long anyway. And if they do sleep, you could always watch a film or listen to the music.
453 by geoff ryman could be good..its a series of stories, one per page, telling about the day of each passsenger on a tube, and lots of them are intertwined. handy if you will be constantly picking it up and putting it down..i really liked it.
this link tells you what each ailine will offer to keep kids entertained..worth checking if yours is listed so you know to ask for them.