Mark of an Angel (2008)

Elsa, a woman with a long history of depression in the midst of a divorce from her husband of 12 years develops an obsession with a seven year old girl she sees at a birthday party when she comes to pick up her son Thomas. Determined to find out more about the girl, Elsa uses Thomas as a way into the girl's family by aiding to develop a friendship between Thomas and the girl's brother Jeremy so that in turn Elsa can then befriend the girl's mother Claire. She uses Thomas more and more in her pursuit of this obsession telling her employer and fellow employees that Thomas is seriously ill so that she can run off watch the girl (Lola) wherever she goes. Elsa even tells her parents lies that she is going out with a friend so they will baby-sit so Elsa can even go as far as hiding in the bushes outside of Lola's house and watching her at night.

Source: TMDb