Kids are weird, largely because they repeat things they hear without any understanding of the meanings and significance behind the words. So in the cases of past lives, they're repeating stuff they've heard on TV, films, documentaries, etc, and describing images they've seen on posters and adverts and book covers. And they talk about it like it's real because at that age, kids can't tell the difference between reality and fiction, so it's all equally real and it all gets blended together in their minds. Then adults read something into it that isn't really there.