Chapter 17 of Clive Barker's novel The Thief of Always is titled "The Bitter Truth." It is here that we discover the bitter truth of Mrs. Griffin's life regarding how she got to Hood's Holiday House.
Mrs. Griffin confesses to Harvey that she was the first child to arrive to Hood's House. She also explains the reason why she is the one who will never leave.
She was nine years old when she entered the house. She got there when she ran away from home, angry at her father for not getting her a kitten after hers died. She was angry enough to wish away many things she had and to wish for things to be different.
Upon arriving, Rictus gives her not one, but three kittens. This is the way they lure. Second, they allow her to be in Hood's Holiday House, which caters to every wish children have, from food to having their favorite holidays celebrated back to back. This is the way they take their souls, as they retain the children in the house for good. Third, she wanted another father, but only out of anger against hers, who didn't want to get her a new kitten.
"And I wanted cats, and a home, and—"
"Another father." She shivered with fear, remembering the horror. "I met Hood that night. At least, I heard his voice."
Therefore, as a first guest in the house, Mrs. Griffin is the one who is bound to stay there forever. She simply chose to try to make the kids happy while they are all trapped in there, and this is why she chose the role of the nurturer who spoils, feeds, and offers a degree of care for the kids.