My personal theory of Swallows and Amazons reproduction is that young sailors get carved out of scrimshaw. Landlubbers, however, definitely might grow from buds!

Mallory Towers definitely doesn't feel sexless in the same way. I think it's because the characters have Strong Feelings about each other, and also the yearning that the girls feel to find their own special friend, whereas the S&A kids are all content to pal around in one big group. There's no pairing off, even of the platonic friendship kind.