Actually,
black body radiation ... is pretty strange and mostly irrelevant here. Never mind.
If you were to seal off the room and make all four walls perfectly
mirrored, then the light would actually stay around. Except, of course, that
you'd be in the room, and absorbing some of the light yourself.
Otherwise... anything less than a perfect mirror will absorb some of the light that hits it. A photon hitting an electron will be absorbed into that electron, energizing it. That's how plants live. Whenever a photon (preferably moving at a wavelength in the green area of the visible spectrum) hits an electron orbiting a certain atom that's part of a certain molecule, it gets "charged," and the plant can use that to help make food for itself. (Which just blows my mind. For the whole thing to work, that one exact atom has to be hit over and over, and it has to happen in enough of those molecules every day to feed not just the cell that it's in, but also those parts of the plant that aren't capable of photosynthesis. It has to happen enough in the leaves of a tree to feed the entire trunk and all the branches, with enough energy left over to last the winter, when there are no leaves... All from photons from 93 million miles away hitting exactly the right atoms.)
Anyway, back to the subject at hand...
Light comes out of the bulb and bounces off a wall.
Some of the photons will pass through the surface. If you hold a piece of paper up to the light, you'll see that, even though it's pretty much opaque, at least a little light gets through. So some of the light is lost that way.
Some of it gets absorbed by the paint (which briefly gets slightly warmer).
The rest is reflected off the wall. That percentage can vary, but let's say that this wall reflects 70% of light that hits it. 30% was absorbed (including the small fraction that passed through the surface to be absorbed deeper). The rest bounces on... to the next wall, where another 30% is absorbed. Then it bounces again, and another 30% is absorbed. And so on.
Some will also go out the window and through the cracks and all. But even if there were none, the walls would eventually absorb (pretty much) all of it. As you noticed, at the speed it's going, it won't take long at all for that to happen, even if it takes a thousand bounces.
You, of course, will also absorb some of it. Into your eyes, where the information will be transmitted to the visual cortex. But also into your skin and clothes. And any other objects in the room. Including the lightbulb itself, which isn't perfectly clear or mirrored, either.
As for what a six-year-old would say... I'd be interested in hearing it. I'd guess at least a few of them would assume that it flies out the window. Enough bounces, and it will eventually find any crack.
