"Where are they going?"
Translation:Куда они идут?
My guess is that где is only for fixed locations. In the sense of "where are you NOW" or "where is the hotel" etc. (Где ты сейчас and где гостиница) while куда is for a direction such as "where are they GOING". Similarly, откуда is where from and is the word куда + от (from)
I love this! I touched the flag by accident, but wound up finding a detailed answer to my question. THANKS!!