What does the playhouse represent?

The playhouse symbolizes safety for Mimi (and, to an extent, Waldo). When Waldo runs away from home, as it snows, he spots the playhouse outside of a store at the mall and takes refuge inside it. Mimi does the same as she walks disoriented along the roads around her retirement home. She finds Waldo inside after being drawn there, and she believes he is Theo. Waldo shows her the stars on his iPad inside the house, and she feels a sense of happiness and relief in her last moments because of both Waldo and the relative comfort of the playhouse compared to the harsh weather outside.