I'm a co-founder of a profitable 11-person educational gaming company called MindSnacks, so I have a pretty solid perspective on what works in this space.

The key to making IAP work in an educational gaming company is keeping things really simple. We give away one lesson for free and then offer the remaining 49 for a one-time $5.00 fee. If you get greedy and try to squeeze payments out of your user at every turn, you might make a tiny bit more money, but you're certain to lose a long-term customer that might buy multiple apps.

We pay a lot of attention to our cross-use (users with more than one app) and retention numbers. We've found that the more you try to monetize like a traditional social games company, the more these numbers start to drop.