Little Linguists Love Learning Languages
This is where apps come in handy again. Introducing children to new languages via the web and hand-held devices is effective because it turns the learning process into a game. LinguPingu introduces basic Mandarin; First Words: Spanish will have toddlers chatting en español in no time. From Romance to Semitic to Continental West Germanic languages (that'd be Dutch, Afrikaans), help your kid develop an ear for the languages they will hear before you land.
Even if all they come away with is "please" and "thank you," it's so important to show cultural sensitivity and good manners wherever you go. Por favor and efharisto will carry any kid traveler a long way. And you might find that once they get home, they have an easier time saying it in Spanish and Greek than English.