Marginal Revolution informs us that children who brush their teeth more than once a day are less likely to be overweight.
At least the guy who write the quoted text was smart enough to realize that there's no direct connection.
I say it's a class issue. People from higher social classes are less likely to be overweight simply because it's seen as low class. And furthermore, people from higher social classes are also more health obsessed in general so are more likely to make sure their children brush.