My son was born a pack rat.
Actually, I don't think he's alone. Almost every mom I've talked with says their kids are hoarders, too.
I tried for many years to break him of the habit of keeping every little thing he has ever laid his skinny fingers on.
I gave up in 2006.
I gave it another go in 2012 and finally broke through (a little).
I learned money always talks, even with children. I offered to let him keep all the cold hard cash he could earn by cleaning out and selling his toys that received no attention whatsoever anymore. He bit.
We sold a few on the web. We sold some at a kids consignment shop. We donated the leftovers to charity.
My house is cleaner (yea!), I can walk in his room (hooray!), and he has the money to buy more ammunition for airsoft guns.
Our initial stab at decluttering actually convinced him to clean out the purely messy insides of his desk drawers, too. He filled up the trash cans and Mama took the trash out. Score!
Now go tell your children to clean out because there's money in their rooms. (Or bribe them with candy - whatever works).