If you use a sling or front carrier then baby can sleep in it thru the flight i have found.
A dvd player if your 3yo is into, it is a good investment,get new dvd's and keep them for the flight, good deals at the airport or on ebay.
Loads of nappies, if baby has 'runny' poo normally, then add a airline sanitory towel into it at boarding and at each change makes a huge difference.
Alco soap, the water free stuff is useful.
It you use dummies then take a small screw lidded tub or small avent bottle with ready made milton in it, to clean them. Milton doesn't need to be rinsed off tho has a minor taste so get them used to it before hand.
Take 50% more formula than you use normally, they will waste some but often want to feed more frequently.
Chat to the head steward on boarding and it most often proves useful for extra pillows/blankets/treats.
Have a good flight and remain calm, ignore grumbles from others tho, a smile and a 'would you like to try' if either children is upset can either shut up the complainer or give you an offer of help/distraction from another passenger.
Pack everything into zippy bags, ie, 1 for each nappy change, saves on nappy sacks & are much better at reducing lingering odours, clean clothes, so that messed up can be exchanged and not mess up the whole bag, feeding spoons and jars in little packs.