For treating Paediatric cancer in the subject for needing it. This method includes that (S)-(R)-((5- -2- (2 of therapeutically effective amount are given to the subject, 5- difluorophenyls) pyrrolidin-1-yl) phenyl)-[1,5-a] -3-hydroxypyrrolidine-1-carboxamide pyrimidin-3-yls), or its pharmaceutically acceptable salt, or combinations thereof.