If you are travelling by car, plan plenty of breaks. Pack them their own little picnic box with a few favourite treats to munch. Don't take their beloved cuddly. There is no other sure-fire way to ruin your holiday than losing their most precious companion. If all else fails, sing.
Toddlers adore a good sing-song.
Story and music tapes might help. Don't ever let yourself be heard moaning about how ghastly the journey will be. If you stay upbeat and positive, it may well not occur to the children that travelling can be anything other than interesting. If you are flying or travelling by Eurostar, forget packing crayons and drawing books. Most airlines have a free pack to give out to kids, and Eurostar's pack comes in a handy backpack.