Is he trying to bite them? You said that he's attacking them, but what is his actual behavior like? Is he just running towards them, or is it an angry lunge? I've never heard a rabbit making a hissing noise, but I've heard them grunt and breathe heavily through their noses.
I've never had a male bun, but my female used to circle my feet and grunt at me, she was telling me she loved me. My girl bun now, runs right up to me and grunts too and makes different noises. The girl bun now, grunts when I put my hand in her cage to feed her.
When my girl runs up to me, she kind of snorts and her tail is really high up, she looks excited and a little crazy. Is that what your bun is doing?
Is there a way that your kids could be going into the room while the bun is out in the room, and they're messing with him? My son screams at my rabbit all the time, she doesn't care and they aren't ever in contact because of a baby gate, but he yells at her everyday and she just stares at him.
Or maybe he just feels more comfortable and confident while he's in the cage and feels scared out of the cage. But I can't say that I've heard of an "attack rabbit" before, because they're prey animals and when they get scared they hide or get really low to the ground. Maybe people DO have rabbits who attack, but I've never seen it with the two that I've had.