As Ruan Chun Xian said, this should be part of canon. It fits so perfectly that it's hard to remember that it isn't some of Lewis' lost writings!
Where to begin... First of all, Little!Frank is just so adorable. I love his voice and the way he thinks of people (girl Mans, Mans with fur on their faces, etc.), how he talks to animals and has a way with them, how he asks if the trees are safe, everything. His thoughts on the Lord's Prayer are hilarious. But also when I was reading it, I wanted to cry because he's so sweet and innocent and he doesn't even realize everything that's happened, to his mother, his brother, his whole country. Sniff. And of course it's just pure brilliance that he is actually the Pevensie's father. How awesome is that?
The Telmar thread of the story is really fun; I love Chrysophylax, who is so different from the evil Mylophylax. Chrys comes off as a mild-mannered dragon who is mostly interested in 'shiny things', but you can sense that he can be very fierce, especially in protection of Narnian Bunnies :D I love how you built the culture of the Telmarines (and how Belisan says, "Ahoy, King of Narnia." He just sounds so lazy and pirate-kingish). Peridan and his mother are wonderful; I love Gree's way of talking and I love the 'dialect' that you built for her. I am just really impressed with how much you built Telmar into such a separate culture, kind of pioneer-pirate combination. Just lovely.
And then the thread of Queen Althea and her two sons. It's really sad and dark, and yet infused with the hope of restoration. I love Althea and her faith in Aslan.
I love the depth you've put into the history: how it was the Narnians that ended up destroying the Tree of Protection, how there was great unrest, how Jadis was looked upon as a second Swanwhite, the prophecy that Timeseer gave that still a hundred years later had partially survived.
There's really not enough good that I can say about this story! It's just about perfect, and I love it to pieces :D