One time I ran a "make your your video game" class at the local library. It was intended for teens, but some parents ended up bringing their young kids. The young kids ended up needing a lot of help from their parents or older siblings, but they did seem to catch on to setting up actors with behaviors, laying out a scene, etc. Someone with more experience with kids could have done a better job than I did with them. (I teach at our university and so I'm used to working with adults. Effectively teaching young kids requires a skill set I don't have).
My takeaway from that experience: 8-12 is a wide range. If you want to focus on programming, Scratch seems like the better choice for the young kids. Older kids who are growing out of Scratch will probably welcome Stencyl's block interface with its expanded range of possibilities but familiar interface. If you have some advanced kids, maybe introduce them to Haxe?