Children's learning is basically limited to what is currently happening. They don't have much control over it. Adults on the other hand can make things happen. Adults can create optimal language learning contexts. Adults can structure events so that language learning can take place.
Language learning for children follows a natural developmental process.
Children go with the flow. However, adults can accelerate that developmental process for themselves. Adults can learn more quickly. However, adults tend to limit themselves to certain ways of learning. Adults are often subjected to a process that makes language learning more difficult than it needs to be.