Children learn a language very fast. They learn the correct accent and intonation by listening and imitating words and sentences. They can also correct their mistakes very fast and they normally do not ask questions as opposed to adults, who always want an explanation and want to learn grammar rules. They just accept what they hear and do not ask question about grammar. They notice it when someone speaks incorrectly. For example, if someone says, "this is a egg". They readily notice the mistake because they have only learn ".. an egg" and "a egg" is abnormal. This is my view of how children learn languages which may not be a scientific one :)