Some yellow sticky notes mark places in travel books and give the kids something to draw on a long bus ride (you can still use them to mark places in a guide books once drawn upon).
For semi-older kids (to include adults and children currently stuck between dependence and responsibility in age), putting coins into those plastic 35mm film canisters makes it a bit harder to lose money out of pockets when sitting. The canisters also help in keeping your lire coins separate from your francs and make it simple to have change handy on buses and trains.
A small compass entertains and enlightens; the little cheap ones are fine to figure out which way the station will likely be (you're not navigating up K2, or if you are stop reading this and look into something much more detailed). You can buy a cheap compass in interesting places, such as the one we got in Japan with Japanese characters on it instead of the usual N-S-E-W markings.
For older kids (or for younger ones if you do it with them), maps they can mark up are fun (where we want to go, where we were).
A day pack the right size for the children to put the stuff they accumulate along the way into (as it gets heavier you get an important travel lesson"Ã¢â€š¬things get heavier by the end of the day"Ã¢â€š¬included at no extra charge).
Pen and paper (perhaps a small notebook with a cover that appeals to your child and thus has a better chance of not being left behind) are handy to make notes, to help a kind English-speaking stranger write down directions on (especially in situations when you find someone nice enough to write directions in the host language down in the host language for you to show to a taxi driver who only speaks the host language you don't speak) and of course to help entertain the kids on long train rides or on the plane.