The Return of Tommy Tricker (1994)

Welcome to the world of stamps with Tommy Tricker and friends as they learn the magic of travelling through time and space for the mere price of the postage. Though of course, nothing is as easy as it seems as they soon learn when they try to rescue Charles Merriweather, a stamp traveller frozen in time since his last attempt at stamp- travelling, 65 years ago. But Charles turns out to be a girl, his sister Molly! And she is rapidly turning into an old woman. To find a cure, Tommy takes off in a race against the time that leads him and Molly to the exotic and magical South Pacific islands.