Depending on your point of view, this is either #3 or #5 in the Narnia series. Chronologically inside the series, it falls during the reign of King Peter etc, which makes it #3. But it was originally published after The Silver Chair, and it's not really part of the arc with the Pevensie kids. It's the only one set entirely in Narnia and featuring children from that world. Lots of interesting allegories here, of interest to adults; the kids will enjoy it as it stands.