Do not be seen by humans. That's been the law of children of the underfloor.
full movie, with english subtitles online, download for free, streaming, watch online. The Secret World of Arrietty (2010)
14-year-old Arrietty and the rest of the Clock family live in peaceful anonymity as they make their own home from items "borrowed" from the house's human inhabitants. However, life changes for the Clocks when a human boy discovers Arrietty.