Cross-Platform Gaming App Development

I'm seeking an app developer to create a gaming application for both iOS and Android platforms. The game will include an achievements and leaderboard system.

Key Requirements:
- Develop a gaming app for iOS and Android
- Integrate achievements and leaderboards features
- Very simple, meant for kids to be educated via gaming (so the kids will take lessons that then tests them, and they collect points and at the end of each month these poitns reset)
- Not meant for monetization, everything is free and no ads. No in app-purchases so easier code I assume
- I will need to be taught how to update the app every month with new material independently without the requirement to hire a coder another time (that's going to be part of my offer, non-negotiable).
- Preferably offline app (there will be no user to user interaction)

Ideal Skills and Experience:
- Proven experience in mobile game development
- Strong knowledge of both iOS and Android platforms
- Experience with achievements and leaderboard systems

Please provide a portfolio of previous gaming apps developed and provide your average price you'd charge.