You would be amazed of how many children and youngsters speak one or two foreign languages fluently, especially those leaving in a non-English country. English is obviously extremely easy to learn, since children have contact with it all the time, through media, video games, the Internet, toys and so forth. However, a child can easily learn a secondary foreign language, or if his mother tongue is English, a main foreign language. It has been proven that we have the best assimilative skills during our young years and it's then that we should try to build up on learning a foreign language.
Obviously, if you're reading this you've probably passed that "perfect language learning age", but if you have children of your own, it might be a good idea to give them the chance to start up on a foreign language while they are still at their learning best. It's important to know that with each passing year, his ability to assimilate information naturally gradually decreases, because he starts gathering up on day-by-day information which fills up the free spots so to speak. It has been argued that the best period to introduce a foreign language to a child is in his first year of life. Bilingual families (where parents have different nationalities, or when both parents are of the same nationality but live abroad their home country) use this technique reflexively, since the child "picks up" on both languages at the same time.
At around age 2, your child enters the "vocabulary explosion" period. During this period that lasts around 1 year, he gathers new words like a vacuum cleaner. Obviously, the main vocabulary will be of his first language, but if you allow him to play with some toys that use a foreign language, especially toys that use images correlated to words (flashcards for example) will also give him a heads up on this new language, even though he might not distinctively know which is which at first. It's important that he uses these words, he memorizes them and he's familiar with them. This will be extremely useful later on when he'll start the actual study of the foreign language you're trying to teach.
At kindergarten age, your kid will already be fluently speaking his mother tongue (maybe the languages of both parents in a bilingual home) so some parents choose to find him a specific kindergarten that focuses on a foreign language. You'll undoubtedly find kindergartens that teach in Spanish, French, German or English (for non-English parents of course) for natives of these languages or simply for parents that want to teach these languages to their children from a young age. If possible, try making sure that the teacher/s of these kindergartens are native speakers or are well-versed in that particular language, or your kid might end up learning it with the teacher's grammatical flaws, pronunciation mistakes, chaotic accents and so forth.