Another vote for Turbo Pascal! It was my first language as well and it really set a foundation for me. It was easy to pick C/C++, Java, etc. afterwards.

I think the kids must have genuine interest of computers in order to learn programming, or you have to somehow generate that interest. I picked up programming in high school, and I thought it was fascinating because I was already very familiar with computers. However, if I were to learn programming in 6th grade when I know nothing about it... I think I would be both confused and bored.

It may be a challenge for you if the kids aren't too familiar with computers (I guess it's doubtful in this age...)