hildren acquire language quickly, easily, and without effort or formal teaching. It happens automatically, whether their parents try to teach them or not.
Children acquire language through interaction - not only with their parents and other adults, but also with other children. All normal children who grow up in normal households, surrounded by conversation, will acquire the language that is being used around them. And it is just as easy for a child to acquire two or more languages at the same time, as long as they are regularly interacting with speakers of those languages.