Before 7th and 8th grade, children have a lot of trouble with abstract thinking - they are much better at concrete thinking. My wife teaches algebra to 8th graders. To get into the class, they have to pass an "Algebra Aptitude test". To see if they have the required abstract thinking ability. About 1/3 will pass the test.

I would think that programming would require a similar abstract ability, so you'll have to be careful either with the content or with student selection.

When you get it up and running though, I bet it will be a lot of fun!