What we started doing when my son was about the age of your kids is having him give away toys. We explained to him that he had a LOT of toys and that there were kids in the area that wouldn't be getting toys for Christmas and that he could help them get toys for Chrismas. Starting the day after Christmas he would pick out 1 toy (or 10 small things like matchbox cars) each day for 2 weeks. At the end of the 2 weeks we, including him, would take the toys down to the local Sharing House where he would give them to the people to give out for Christmas. The cleaning out has gotten harder over the years becuase he has given away most of the things he didn't like. Now he has to make tough decisions and really think about the other kids. He's actually always done that, some of the toys he's given away in the past have really surprised me.
We also started having him make a Christmas wish list, explaining that you do NOT get everything on your wish list, it is a list of items you WISH you could have. We also had him rate it from what he wanted most to least.
Between the 2 we got a pretty good idea of what he did and didn't like.