[vba616]

Erasmus Darwin wrote in "A Plan for the Conduct of Female Education, in Boarding Schools":

   "For the drink of the more robust children water is preferable, and for the weaker ones, small beer..."

Sounds rather...Darwinian.

[animal531]

That's perhaps from the days when water wasn't quite as clean as it could be, and other drinks were preferred.

[throwaway984393]

Beer is basically liquid bread, so it's nutritious (or at least has calories). Small beer has low alcohol so you don't get drunk