We flew quite a bit with DS when he was that age as family live at the other end of the country and 12 hours in a car isn't my idea of fun! We also flew long haul a few times.
The things we found helpful (and most have already been mentioned in some form)...
Explain about the airport so the older one isn't overcome or worried. We used to call the security pat downs 'tickles'. So dad would go through for a tickle so DS could see what happened and one of us would be at either side of security in case he did a runner! Then DS would go through, then me. We'd explain that the tickle didn't really tickle but you had to put your arms straight out, like a plane and stand very still until the man or lady said you were done.
DS's job was to tell us when our hand luggage came through from the xray machine too (he often missed it so don't rely on that!)
Take lots of stuff to keep them amused, from snacks to books, colouring books, buy a magazine at the airport too as they love the idea of buying something 'special' there. I always put things in a separate bag for DS but I'd control what was taken out and when - he had a habit of unpacking the lot and the 'surprise' element would be gone.
Take a soft toy so they can cuddle up if necessary but make sure it's packed away well before landing. DS used to help put his 'to bed' in the bag.
Make sure the oldest one understands about the noise on take off and the 'push back' you feel as you go along the runway - that always makes me a bit nervous even at my age.
Oh and don't forget wipes in the bag - plenty of them. We were on a flight once when a child in front was sick. They had a change of clothes but mum was caught and the child had it all down their leg. Luckily we had loads of wipes so could help out.
Plenty of suncream, a tshirt if they're in the pool and something for their feet as the ground may be hot.
Then relax, don't worry about sleep times if you can't keep to them. Just have a buggy so they can nap if they need to. And have fun!