Barberries? Really? That would teach the grandkids to keep away...

Seriously though, the only thing I could suggest given your restrictions, would be three upright panel railing sections around the u-shaped periphery of the window well frame. Plexiglass would let in the most light and probably be the least intrusive visually. They're readily available, not too expensive and would be pretty easy to install.

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I had to get out from a similar place once during a minor fire, and shove a panicky young child and a yowling scratching cat ahead of myself.... I don't think I would have wanted to deal with the barberries [wry grins].

If I were you, since light access is important, and wood might be an ok option, I would be looking at something like this.... Hard to describe....

You want a wood 'grate' that will keep the kids from accidentally stumbling into the well. Since the tops of the windows are roughly at ground level, is there a way you can install a 'stop' at the top of the window? Even a piece of 2x2? Or, alternatively, some sort of upright at each side of the window that can serve the same purpose?

Then outside the curving metal at ground level, make three sides of a square. 2x4s? Anchor the corners solidly.

Then get an open wood lattice, or make one. Hinge it to the outside square, and let it rest on the stops.

Won't keep out burglars, but it will block falls. And, from the inside, you can open the window, shove the lattice, and it will open on the hinges and land to the outside, and be out of your way. It will let in lots of light, and catch plummeting children.

Hope this makes sense. I hate trying to describe the 'picture in my head' in words....

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Ummmm...are you certain this is really a problem? We have window wells...and we've had kids here (though they are now grown)...and no one ever fell in. Truly.

If it really IS an issue...both linda s's and adriennemb's suggestions should work.