In reply to Green Porridge:
I first flew aged 7 and it was FUN.
Explain to them what to expect at security and how to behave if they need a pat down or to have their bag checked. Warn then to expect lots of boring queues and waiting around.
At the airport go for a walk and do a bit of plane spotting. See how many different countries carriers you can spot (Singapore, Egypt, Iceland, France etc) from the aeroplane livery.
Whilst it pains me to pay extra, do ensure you are guaranteed seats side by side with window. Don't expect other passengers to swap and at that age I would have absolutely hated to be sat with a stranger. Don't tolerate them bouncing on their seat or kicking the one Infront, both of which are infuriating for other travellers.
Embrace technology with a kid friendly film or whatever on phone/tablet (don't forget headphones!), also colouring books or other non battery entertainment. An adult fleece makes a lovely fluffy kids blanket incase they want to sleep on the plane.
Bring an empty water bottle through security (lots of water fountains airside) and pocket tissues to mop up any spills. Try to avoid overloading then with fuzzy drinks and crap, instead try to choose healthier snacks that are less likely to fuel any travel sickness.
On that subject, if in doubt do dose them up on travel sickness remedies BEFORE travelling. Also check the seat pocket for a paper bag and make sure they know what the bag is for, just incase.
Talk them through takeoff and landing make it fun rather than scary, bring boiled sweets or chewing gum to help with equalising their ears
Post edited at 19:43