Every year the same families come back to the lake. Every year, the children are older. Every year, they pass through the same sort of behavior that other kids did the previous year.
According to this narrator, kids fall into various sorts of behavior, and how they behave at 2, 3, 5, or six years of age is reasonable indicative of how they'll behave a teenagers. I'll go even further: it's how they'll be when they grow up, G*d willing and the creek don't rise. How they learn to deal with each other, whether they're happier on their own or in groups, things like that, this short film informs us, begins at a very young age.
And all parents worry. That also begins at a very young age.