To Free or not to Free, that is a very good question!
Unless you try you'll never know. And even then, every app is different. There's definitely a market for games like this - I would buy it (if my son wasn't 14 and doing his GCSE's). It's simple, obvious and surprisingly good fun for an educational app. I'd highlight that fact - it actually makes learning fun.
I'm not sure how you should release them though - all in one go, or one at a time. Another decision for you Gernot!