I came across "Jammo" in extras-testing, a new music making and singing app for kids, made specifically for the N900.

Homepage

QA page for version 0.5.0-1

The digital mentor in the game is a bear, who only speaks finnish for now, but you can read the translations in the user manual

Just thought I'd plug this new interesting application. I haven't tried it yet. It's still downloading the 284 MB...

ps. Only experienced users should enable extras-testing.