“Come down!” means the same thing as “get down!”, if both are used in the sense of yelling at a child who has climbed on top of something he shouldn’t. But what's the difference?
While the two phrases do essentially mean the same thing, in colloquial speak, the former, "come down!" is perceived as somewhat less forceful than the order or command of "get down!" That said, if a parent is yelling at a child, both will be viewed as equally forceful.
But take another example: say you're on a hike with a person, and they happen to be at an elevation above you, and you'd like them to join you on a lower vista to get a better few, you might holler over and say "Hey, come down here!" as opposed to "Hey, get down here!" The former is more of an appeal to the person, rather than an order.
"Come down" is oriented with respect to the speaker: 'Move down toward where I am'. If the speaker were above the listener, "come down" would be the wrong thing to say.
"Get down" is oriented with respect to the listener: 'Move down from where you are'.
Note: There are also other colloquial meanings of "get down" (dance, duck) and "come down" (sober up) but I'm ignoring those above.