Are you moving to a new country, or want to pass a language you speak on to your child? Maybe you are already bilingual, or perhaps you are going to be working on your child's second language. The question is how do you teach your child a new language?
Teaching small kids foreign languages is big business, especially in countries where English isn't the official language. If you speak the target language fluently yourself, you have no reason to pay lots of money, and you have a much better chance that your child will end up being proficient too. Yes, even if you don't know the formal rules of grammar.Immersion, being surrounded by the language you are trying to learn, is by far the easiest way to acquire a new tongue.
My bilingual children started to speak a third language this way. They never sat down to formally learn it, but watched videos on YouTube, watched TV, talked to relatives on Skype, and had their mother speak only the new language with them until they started speaking it, too.
With this method, both my children went from speaking zero words to being near fluent speakers in a little under a year. Because they were already exposed to the language before they started speaking it, it came easily. But immersion is a wonderful way to learn a new language for anyone who has the opportunity to do so, and especially for children.
Those who are moving to the country where the new language is spoken by everyone are even luckier. Children who are spoken to in that language by lots of native speakers throughout the day basically have no choice but to start understanding and speaking it very quickly.