When child actors act well they're just reacting to situations, and they're acting very real because their life experience is so short; there's no history to fall back on.
When I was 10 years old, I loved - I loved books, and I used to haunt the secondhand bookshop. And I found a little book I could just afford, and I bought it, and I took it home. And I climbed up my favorite tree, and I read that book from cover to cover. And that was Tarzan of the Apes. I immediately fell in love with Tarzan.