Was watching other stuff yesterday, but yep, watched it in the
cinema.
The ghost is actually envy of the child for having a mother to look
after her, and decides to "trade" the child back for a mother where
he will be "eternally" looked after....
yah...quite a selfish ghost, but can understand, cuz a child ghost
also not very mature and will be jealous....so heck, the mother is
sacrificed. 
