How does Lillian convince the twins to come with her in Chapter Three?

In Chapter Three, Lillian convinces the twins to come with her by telling them about the beautiful swimming pool at the estate. She asks them if they are really happy staying with their grandparents, and Bessie admits they are not. Finally Lillian promises to care for them, buy them things, feed them, and entertain them. When she promises to love and comfort them, the twins agree to come along.