This isn't an ending for Adam that I'd ever thought of before--that he might continue to exist as his child self, powers intact but never growing. Then again, it's hard to imagine Adam grown up, so. Yeah.
Erm, other than that, I like how you made the crossover work--and how Adam gave Mal a happy ending while still keeping to his "people need to help each other!" creed. :) Annnd...uh, I like how it wasn't revealed why exactly Mal might be lonely and missing the Serenity until the end. That's a nice touch.