Clear plexiglass, custom cut to fit over the top of the set and extending all the way to the bottom. Downward facing slit and pointing away, for the speaker section.
I'm thinking of a curtain that could be pulled in front of the screen when not in use, and pulled to the side (for sound and light performace) when in use.
Are you worried about them getting it dirty by touching them or are you sorried about them poking it with something?
I know in my case I went with a flip up screen because of space, but also because of my little kids. I bring it down to watch movies and then flip it up when not in use. When they get older perhaps I will buy a real screen.