Yep, handheld devices are great (download FREE classic books onto their devices before you go -- you can find them online) but be sure you have some other forms of entertainment that involve their hands -- kids this age need to do that as well; it engages a different part of their minds. If they do word find puzzles, crossword puzzles, sudoku, any other puzzles, that's great (you can find tons of free, kid-appropriate puzzles online to print out for them). If they like to draw, bring small ringbound drawing pads and good quality colored pencils and sharpeners (do not forget the sharpeners)! Bring real books as well as electronic ones. A portable game they both can play together like a magnetic travel checkers set, if they play together OK. Some form of small craft -- "finger knitting" is terrific because all you need is yarn and nothing else (maybe some blunt kid scissors but check with security).
Remember too that the airlines now have TONS of programming on their in-flight video systems, not just one lousy movie. There is likely an entire channel of all-kid programming for both video and audio, depending on the airline. Take care that it doesn't end up kind of mesmerizing them -- I limit my middle school daughter's watching stuff when we're on the plane so she's not glued to her seat. It is very important that they get up and down at times. And some content may not be what you want for your kids, even on a kids' channel.
Keep them hydrated and walking periodically. You will have to get bottled water after you pass the security checkpoint so do not bring any liquids from home or get them before security; you'll have to dump them.
Bring those germ-killer Clorox wipes -- forget the canister, put them in a ziplock bag. Airplanes are dirty no matter how much they clean them so I always clean the seat armrests, headrests and tray tables, and carry wipes into the bathroom as well to clean there (you have about 50 people per bathroom on a large international flight -- think about it). You don't want the kids doing this cleaning on their own, though.
They and you should avoid greasy or heavy or high-carb foods the whole day of the trip -- you will feel worse if you eat those things and then fly. Eat lightly and have them do the same. Drink water and not soda -- soda is actually dehyrdating! Some fruit juice is good for a pick-me-up when you're close to landing and they have to be more alert to get off the plane, through baggage claim, etc.
We fly to England every year and have since our daughter was born 12 years ago. I hope you have a great time.