Maybe run a series of articles and include the parts needed to build the actual robot. Much like Real Robot's Cybot -
https://www.google.co.uk/search?q=cybot ... s&tbm=isch
Something simpler though, cheaper and over fewer issues. Cybot worked out quite expensive if one went all-in but just a few issues and £20 got a decent chassis, motors, motor driver board and battery boxes which was enough to have one's own micro controlling it - It got me into building my first ever bot.
Alternatively a partnership with one of the official retailers who can sell the parts to make an "Official Raspberry Pi Robot" might work. It is having a cheap platform which is 'plug-and-play' which is the biggest problem for most people.
[ Cross posted with B.Goode's post - That CamJam EduKit#3 looks great to me but think it should come with a chassis ]
With a suitable platform, and a few simple sensors, it's then all software. There could be a tie-in with Scratch and/or Blockly so could be well suited to educational Pi users.
It should also encourage more subscriptions because issues with 'parts included' aren't going to hang about on shop shelves.