I have three younger children (8.5, 7, 5.5) and they're all VERY intelligent (straight A's in school, brilliant product ideas, etc)....
They come from a "broken" home, in that their Mommy and I are divorced. Sometimes it seems that each of our houses has different rules, even though they don't. I truly believe it's the discipline, "threats," and follow-up (or not) of those "threats" (like clean your room or else) that seems to be off balance at each place of residence.
My kid's room is clean at my house, but at their Mommy's apartment, they're trashed. Granted, they live w/ Mommy most of the time, with me getting visitation. But still....
She claims that there's no room to put things at her apartment, and they have too much crap. While I agree that's true, it's still necessary to get the kids to clean their rooms when all there is from the door to the bed is a minefield of toys, papers, shoes, etc that they have to walk on. Many a foot has been cut or poked w/ toys on the floor!!! And it's not like the kids can't pick and choose what to get rid of: they did that at my house and filled three Xerox boxes with unwanted books and toys!
My Ex has threatened to give some of their favorite toys away to charity if they don't clean their rooms. She's had her boyfriends help clean their rooms for them, where they just had to help w/ the upkeep of cleanliness. She has spanked and taken desserts away from them not cleaning their rooms....
Yeah, OK. That all worked for about 5 minutes.
My kids don't care about spankings anymore. They don't care about no dessert (they're becoming more health nuts anyway).
Anyway, I told them what my fiancee's Dad did to her when she was young and refused to clean her room:
Her Dad waited until she went to school, then put ALL her toys and books and music and everything that was in her room into boxes and threw them away. Yeah, as in "into the garbage."
Yeah, he got rid of ALL of her stuff, as a punishment for disobedience. He never yelled. Or spanked her. Or anything. Just got rid of all of her stuff in her room since she refused to clean it up. Then, she had NOTHING left to make the room dirty and messy with.
So, the next time your kids refuse to clean their rooms, tell them about that. Maybe that'll change their tune. I'm sure they'd hate to lose their games, books, toys, and everything.