"Anna" is actually a disturbed young woman named Emelie. Emelie (Sarah Bolger) was a young mother whose child accidentally suffocated next to her when she fell asleep. She was unable to conceive again and the courts decided her to be a danger to adopt. Meeting a man of a similar disturbed nature, they conspired to find the perfect child to make a family.
Emelie and her partner kidnap and murder the real Anna and Emelie poses as her to babysit the Thompson children. Only eldest son Jacob (Joshua Bair) realizes how crazy she is as she sets sights on taking his younger brother Christopher. Attempts to get help fail, and Emelie winds up killing Maggie, a friend of Anna who was concerned about her being missing.
Needing more time, Emelie tells her partner to cause a car crash to stop Jacob's parents from coming home, but it only results in the man's death, and discovery of the body of the real Anna. Jacob meanwhile had scarred Emelie with fireworks, and has run her over in his dad's car. He is able to drive his siblings to safety before his parents arrive.
The police arrive, but Emelie, despite her injuries, is able to get away.
Thanks Tim (Spectre)!