Towards the end of The Woman in Black (2012), Arthur Kipps finds the dead body of the child in the marshes, and he places his dead body into his grave.
I thought the grave should have been empty, but there was another corpse (in skeleton form) in that grave. Whose corpse is that?
The child's mother.
The reason Arthur went through all that trouble was to reunite the two since her ghost was possessing/killing all the children. Also, Arthur's son was on his way to town.
He'd hoped it would chill her out.
But no, the ghost was just as crazy as the one from The Ring.
Per the screenplay, it was the boy's mother, named as Jennet Humfrye
EXT. EEL MARSH GRAVEYARD. NIGHT.
Arthur and Daily stand in the mist before Jennet Humfrye's headstone, Arthur cradling the shrouded body of her son.
At the foot of the headstone there is now an OPEN GRAVE.
Beside it, a pile of DIRT and a discarded SPADE.
As we follow Arthur's gaze into the hole, we see that Jennet's CASKET is visible.
Arthur hands the child to Daily and jumps down into the darkness of the grave.EXT. OPEN GRAVE. NIGHT.
Straddling the casket, Arthur pries open its lid.
Inside we see JENNET'S REMAINS - just bones now, and shreds of her tattered black mourning gown and veil.
Daily hands the boy down to Arthur, who places the child gently into the coffin beside his mother.
One last look, then he replaces the coffin lid.
