I would rather take the Y seat. It's a small sacrifice to ensure that the 10-year-old is sitting next to someone she knows.
As for trading seats, nobody really cares -- people do it all the time. Your party has 2F + 1Y seat, so if anyone asks, just say you traded.
The only problem I can foresee is the 10-year-old thing. On some carriers, splitting a parent from their child in a different cabin would trigger an Unaccompanied Minor fee. I don't know what AS's policy is on this.