Revealing mistake: Right after Nell sends Hugh to hell, Dr. Marrow and Theo come out of hiding behind a pillar. As Marrow puts his hand on the pillar, it squishes in as if it was made of foam rubber, instead of stone. (01:39:50)
Revealing mistake: When Luke tries to ram the gate with his car, a huge piece of spear-point wrought iron falls and impacts the roof of his vehicle. As the others rush to rescue Luke from the car, we can see the wrought-iron spear points flexing and bending, because they are made of rubber.
Answer: Another possibility is the spirits of the children. As long as Hugh Crain was inside the house, the children were trapped inside too. They could have called Nell because they needed her help in being set free.