Your kids being 12 and 9 are the perfect ages for the HDD. We have been to the HDD 8 times, the first time with our kids 9, 5 and 2 and the most recent time 17 and 14 (the 22 yr old didn't go) and they have enjoyed it every single time.
Every time we go on a family trip to WDW the first thing out of the kids mouths is "are we going to HDD?"
Take your kids, they'll love it!