I was trying to think of pre-1950s evil child (as opposed to Bad Kid) movies
when my wife reminded me of 2 WWII flicks: HITLER'S CHILDREN, with Tim Holt
and Bonita Granville (she later settled down with Jack Wrather to take care
of Lassie), and TOMORROW THE WORLD with Fredric March, Betty Field and
Agnes Moorehead and Skippy Homeier as "Emil"("Watch out for me!" said the
ads).
 
Homeier, BTW, turned up on yet another STAR TREK episode as leader of a
group of space hippies.