Pack a night light - we leave one plugged into the bathroom power point with the door slightly ajar for when you need to get up in the night.
Get small torches for the children - helps with the dark and being in strange rooms. 3 & 4 year olds will love them, you can get teeny Maglite ones (these may be too small & easily lost) but I've recently seen some which have glow in the dark covers. Around $4-6
The cash passport cards are good, you load them up with money, use to withdraw cash from ATM's without the crushing charges for cash advances or foreign currency exchange. Our members can advise, or you can find a thread on this site, about what cards offer the best deals. If a bank can screw you they sure will!
Maybe out of date, but I always get my full camera cards downloaded onto cd - duplicates - and send a copy home. May not be practical though if you have to trawl through towns searching for the local PO.
A previous poster recommended copying all docs and leaving with a trusted friend. I'd also suggest including all your and your partner's passwords for banking, computer, and any other accounts where you need to log in.
The 4 year old could pull a little roller case - Thomas the Tank, etc. Light and easy to manoeuvre. I'd advise you against a Trunki - they are absolute cows to pull along and you will be transiting some fairly lengthy terminals on your travels
Travel insurance, travel insurance, travel insurance. Don't leave home without it
Most important item - pack patience and a sense of humour! Enjoy!