For older kids, it might be a good idea to get them to build the kits themselves. If you use components that simply plug-in, it would let them immediately get "stuck-in" without worrying about soldering irons etc.
I'm not sure how you are planning to set up your club and how willing/able the kid's parents would be to contribute to the costs but if they could then you could get each of the kids to design and build their own Raspberry Pi.
By design I mean choose a particular case and decide what they wish to add to the basic board. It might be a good way for the kids to learn about a wider range of topics. Again, this depends on money but if you had a budget for each unit (whether the parents paid for it or you managed to get funding from elsewhere) they could learn about budgeting, planning and other things. This depends on the age of the kids though but it might be an idea.
As they design and build their Raspberry Pi they can learn the basics of what a computer is and then move onto programming. They might also appreciate it a little more as kids often do when they do something for themselves that they didn't think possible.
I'm not sure what would be the best language to use with the Raspberry Pi right now, I would have to take a look at one myself before I decided that. Personally I would try to avoid languages which rely on big interpretors or engines as these would use up much of the resources on the Raspberry Pi before any actual software the kids might write started running.
Just a thought: Anyone know if smalltalk is still going? I'll have to have a look later as a stroll down memory lane