The text plays on the double meaning of 'pick up.' One meaning is the literal action of a magnet attracting and lifting metal objects, and the other is the understanding or comprehension of something, as in 'picking up' a concept or idea. The humor arises from the child's expectation that a magnet should 'pick up' (understand) the disks, which could also be a play on the term 'floppy disks' that are not magnetic.
Daddy , how come my magnet won ' t pick up these disks ?