Here's a blow up of the area in question:
The boys don't touch the radiator itself because it's hot, but they are digging into the crevice shown above and back behind the radiator is a loose concrete wall. At the point where the radiator pipe emerges from the wall, the concrete is loose and the boys are able to rip pieces of concrete off and play with them. Because I live in an apartment building, I am not allowed to mess with the radiator or that wall and I doubt I could get the super to do much about it (it's hard enough to get him to fix other things). This loose concrete has never been a problem until now and it's only a problem because the boys are going back there.
Obviously, I don't want the boys removing more concrete from the wall and breaking the wall or swallowing a piece of the concrete. What can we do to keep the boys out of the crevice? I'd love to put a thick book in the crevice as we've done with other crevices, but I'm afraid the hot radiator would set the book on fire. If I put a garbage can or other large object in front of the radiator cover, do you think it would be a fire hazard?