We did Backstage Magic last September.
As others have said, the quality and tone of the instruction of the tour will vary quite a bit depending on who you get as your guides.
I didn't pick up a lot of new information I hadn't gotten elsewhere (here, for instance), but as such a longtime Disney World fan it was worth it to finally see the utilidoors and the internal workings of Tower of Terror (or at least the maintenance bay) and The American Adventure. The real highlight of the tour was the all-too-brief walking tour of Central Shops behind the Magic Kingdom. You never know what kind of fascinating attraction hardware will be sitting in there in various stages of disassembly, painting, or repair.
I've looked into Keys to the Kingdom but it doesn't really appeal to me. Seems like there's too much overlap to the actual backstage elements you see between that and Backstage Magic.
That one's kind of a "best of" so if you only want to do one tour at WDW, that would be the one.
Years and years ago when I was 10 or so I took some sort of all-day kids' class through the Disney Institute. It was focused on "art" but tended to bounce all over. We were taken by van to the Magic Kingdom and rode Mansion (no backstage there, to my disappointment) and then to MGM Studios for a really fascinating walk at cubicle-level through the animation studios. We had a chat with a couple of the animators there, then were taken back to the Disney Institute for various art-related activities, drawing lessons, etc.
Before that I did some sort of kids' day camp/tour thing on Discovery Island. I don't remember much about it though I do have a picture the staff took of me with a big white parrot perched on my arm.