Question

Why do children make fewer mistakes learning English than adults learning a second language?

Answer 1

Because when you're learning your second language, sometimes people have trouble carrying over their primary language into the new one. It's more of translating your primary language as opposed to learning a word for something you've never seen or been a part of before. 

Answer 2

Children acquire language more naturally and unconsciously. Adults, on the other hand, have more experience with their native languages, so they are more conscious of the grammatical rules. As a result, adult learners of English or any other language tend to transfer the grammatical rules and phonetic sounds from their own native languages into the target language.