Why is Susan so distressed after she saw Jinny kiss Louis?

As the characters are so young, it would be difficult to insist that Susan's reaction to the kiss is one of jealousy. There is not any great indication that Susan had particular feelings toward Louis, and she did not seem to harbor any ill-will towards Jinny, though there is an amount of distrust there on both her part and Louis's. Her reaction, then, should be attributed more to confusion and loss of innocence. It is safe to assume that this is not something Susan had witnessed children her age do before, particularly not among her friends. Her reaction, to flee and weep, was perhaps a final moment of innocence, as she recognizes, perhaps unconsciously, that from that moment on Susan's life, and the lives of all the other children, will be deeply affected by things which they do not understand fully.