As a stay-at-home-school dad / soccer coach that spends most of my waking hours around a LOT 5-16 year olds, both home and public schooled, my answer might surprise you: I don't think you should feel the least bit bad about having that room off limits without you there. This generation of children have been raised by most of their parents to be rude, self absorbed assholes without a shread of empathy or resepct. Some of these poor little people barely seem to have formed a theory of mind.
Now, if you are a GOOD parent you actually owe it to THEM to let them play with your supervision because it seems most children have never had the experience of personal directive investment. This might be regional, but honest to God, the first time I tell a student, kid or player, "No" is often the first time they've ever heard the god damned word in their sad random life.
People are answering this question as if you're wondering if you should ever let them be touched (which would make you a dick head), you're not. It's perfectly reasonable, from what I've seen of the children being produced in this culture, to mandate your own supervision. Or yeah, wouldn't surprise me a bit to see them hanging off plungers like a monkey bar (or actively TRYING to break shit, you know, because), playing around underneath them yanking on cords, throwing shit at backglass, OR, Jesus Christ, these things are clad in metal armor... You do NOT want some hyper indulgent Disinfectant Mommy calling you in a panic because Connohr comes home with fat ass bumb and scrape on his forehead from getting rough housed into the corner of one of your pinball cabinet's by Aeideon.