I drew photos for my 5 year old on what to expect at each point, eg:
drive to airport
(check bags - if you have any)
go through security - show tickets and take off shoes, funny beepy machine
have a sit down/wait for the plane (I bought a new colouring book with me with stickers and felt tip pens, a godsend)
queue up to board
board
wait a bit
fly
land
get off
go through passport control
(get luggage)
Onward with train/bus etc.
It kept him very interested in his surroundings and what was going to happen next.
You could make a bingo sheet of things you might see: pink suitcase, wind sock, sniffer dog, luggage transporter, pilot... Then they could make themselves one for the journey back. With drawings, not words.
I spy is also good, as PP said.
Cokouring books, sticker books, playmobil characters all useful. An aunt gifted DS with a playmobil policeman kit that came in its own little briefcase type thing, which helped on the flight as it was self containing, no little bits of lost toy on the floor.
Sweets to suck to help with air pressure a re a must - chupa chups are good.
The older one will be super excited, so lots of patience and keep chatting.