For fan re-reading, and the 13-year-old, I'd go for internal chronological order (without obsessing about the framing material from Borders of Infinity, which in any case isn't in the ebooks on the Cryoburn CD). For the 10-year-old, I'd start with The Warrior's Apprentice and let him work through Miles in internal chronology, and then backfill with Shards of Honour, Barrayar and Ethan of Athos if he wants. I'd skip Falling Free in either case.