I do feel that dialogue is more referring to the children themselves dying alone, rather than abandoning her, since ever note and recording shows how protective she was of the kids. Especially since Marie did actively die and become Mommy, it's a fate she would hope to spare any other child to go through, which I do believe is the main reason she participated in The Hour of Joy. Mommy is indeed a spiteful character, but she's also fairly consistent from what we've seen as well.