Read first, or watch first?
Watch first, and enjoy it — this is one of the few books here where the film being unfaithful barely costs the reader anything, because the two are not competing for the same ground. Disney's 1960 version is a good adventure film with a plot; Wyss's novel has almost no plot at all, and is instead a decade of small competent work described by a father who cannot stop teaching. A kid who has seen the film will come to the book already loving the tree house, and will then be startled to find it abandoned by chapter nine, a fourth brother who was never in the film, and a snake that gets the donkey. Those surprises are the pleasure. Pick the 1960 film; the 1940 RKO version is hard to find and Disney kept it that way on purpose. If your family gets on with the book, the thing to do afterwards is watch for the pirates, and notice that they never come.