The most usual way for children to grow up being fluent in more than one language is to have a situation where you speak one language at school, outside the home etc and another language in it (usually because someone at home is a native speaker of a different language). Where this doesn't happen naturally, your children are unlikely to grow up really fluent in several languages. To give them the best possible start, you can do the following:

Get them contact with someone who speaks a language you want them to learn. Think about an au pair, an exchange visit , Skype classes or even a private tutor. Try and get them interested in having a pen pal with another language (you should check pen pal sites carefully if your children are young, of course). Let foreign language films, internet and radio programmes be a part of your life.

Try and take holidays in areas where little English is spoken and get your kids to try speaking the local language. Take them where they will meet other children - they will learn from each other really fast. Above all, emphasise that learning languages is fun!