There are several reasons why children learn a language much faster and experience much less difficulties acquiring a new language than adults.
1. The basic rhythm and certain features of a language can be heard by the baby inside the womb. This is why babies prefer to listen to their parents' native language(s) when they are born rather than a foreign language and they also prefer to hear their mother's voice than another female's voice. They start distinguishing languages before they are even born.
2. Connections between neurons are responsible for the acquisition of language. These connections become stronger during the learning process. The more we use our first native language, the harder these connections become. When we learn a new language we often use those same neural pathways and the new language competes with the old one. When these connections are still "flexible" information can be stored much more easily. This can be a help when learning new vocabulary. Children acquire vocabulary much faster than adults. Between the age of 2 and 6, they learn 10 new words a day and many after hearing them only once or twice. Starting at the age of 6, they start learning up to 20 words a day.
3. Children are less self-conscious about making mistakes than adults. This is why it is easier for them to use new words and sentences. Typically, adults are conscious about the fact that it is naturally harder for them to learn a language than for a kid. They keep repeating this to themselves and create a huge obstacle in the course of their language training.
4. Children usually make use of a smaller pool of vocabulary while adults think and communicate in a much more complex way. Children usually use language to communicate their basic needs while adults use language in many more situations and to solve complex problems. Consequently, adults take longer to use a language effectively.