1950s-70s children's book; children on island riding tigers; protagonist called Dar(n)ley or similar; scary monster ("Wambaata"?) on far side of island
I remember the German cover (the blond-haired protagonist riding a tiger, joined by the girl on hers); the boy sets sail in his boat down the river onto the sea to live on an island inhabited almost entirely by a group of kids (with two adults, a man who mends their fishing nets and a woman who cooks). Together they ride tigers through the jungle or swim in the sea. Over all they do, there is the threat of a monster ("Wambaata" or similar) that lives in the crags on the far side of the island. One of the girls decides that she wants to learn "grown-up" things like maths, and seeks out an old wise man living in a cave on the main mountain of the island. The protagonist (Darley or Darnley) turns out to be the one capable of slaying the monster: he has a torch that does not go out under water (his shiny blond hair), and wields a wooden sword (a broken-off paddle). The entire story ends on a bittersweet note, which I loved as a child. I remember finding it thanks to Google about 10 years ago, but the search functions have become so horrible since then that it has functionally vanished from search engines unless you know the name.