PS: Lexxity's advice should be taken into consideration. The lady knows what she's talking about when giving advice, her little one is an absolute angel inflight and not only.
You're embarrassing me.
The tip about the tray table is right. Even if it's in the armrest it could drive you crazy. Put that on the "only Mummy and Daddy" can do list.
Another tip is car seats. We always bring ours and the only airline that's refused to lets us use it was Virgin. On the 330 though you would be restricted to putting it in the window. The 767 is better for this as it's 3 in the middle.
Something you might want to get for your little ones is
These. They are fab. They fit little heads and are noise cancelling! We have made great use of ours. Portable DVD's are worth bringing just in case. Leo get's bored with some of the inflight stuff so we have Thomas on standby.
Before you get onboard try and find a quietish spot in the departure lounge and let your two run around like mad. Get the energy out!
The sugar free thing is a good idea, but small little chocolates will also help in diffusing a situation with a four year old at 30,000ft. I know of what I speak! For take off and landing try lollipops for your older one. Something to suck on if they can't get the hang of holding the nose and blowing to clear their ears.
My last thing I can think of is get off last if you have time. That way you can calmly pack up and get your gear, but more importantly you can let the kids have a run on the way to immigration.
BA gave our son a nice little book in the inflight kiddie pack. "My First Flight" by Usborne books. I can't see it on their website, but if you can find it then get it. It's very nice and aimed at smaller children so can help explain the whole fliying gubbins.