I think most kids who read quite a bit or watch TV much have seen photos of the Coliseum.
But that's not really the point.
The point is that you need to include enough details to paint the mental picture for the reader, without losing the voice or the forward movement of the story.
It's a balancing act, really. Too much description slows the story down; not enough, and we don't feel like we're really there.
If it's a new setting for the main character, someplace she hasn't seen before, then you need to show us the details that strike her as interesting or important. And you need to use those details to set the mood or tone of the scene.
Is it huge? Does she feel overwhelmed by the sheer size of it?
Is it in ruins? Does she feel sad thinking of the things that used to take place there?
Is it threatening, or spooky, or cold and sinister?
Use the description to create the image you want the reader to have. Whatever is important to the story needs to be shown. So it depends very much on the story.