'It' is Psanmead, an ancient, ugly and irritable sand fairy the children find one day in a gravel pit. It grants them a wish a day, lasting until sunset. But they soon learn it is very hard to think of really sensible wishes, and each one gets them into unexpected difficulties. Magic, the children find, can be as awkward as it is enticing.
Eddie Izzard....Psammead ("Voice of It")
Filmography links and data courtesy of The Internet Movie Database.