Cristina Moreno (Shelbie Bruce - Aimee Garcia as narrator) is applying to Princeton University. For her application essay, she tells the story of her childhood and narrates throughout the movie.
Flor Moreno (Paz Vega) is a poor Mexican single mother who moved to America to have a better life for her and her daughter, Cristina (Shelbie Bruce). When she could not maintain her two jobs due to the safety of her daughter, Flor's cousin takes her to an job interview as a nanny for the Claskys, consisting of John (Adam Sandler) and Deborah Clasky (TĂŠa Leoni), their children Bernice (Sarah Steele) and Georgie (Ian Hayland), and Deborah's mother Evelyn Wright (Cloris Leachman). John is a man who cares about cooking good food and raising his kids. Deborah is a former businesswoman turned stay-at-home mother, and Evelyn is a raging alcoholic. Deborah is uptight and her neurotic behavior often upsets the family: she mentally abuses her daughter to exercise by buying her smaller-sized clothes and putting her down for certain behaviors, and she mentally abuses her husband by telling him to co-parent with her on their son, but she really wants him to be submissive to her parenting instead. John is more laid back and supports the mental well-being of his children, but he feels he cannot stand up to Deborah on her parenting and often leaves it as it is.
Summer comes and Flor is needed 24/7 at the Claskys' summer home. Unable to communicate well in English, Deborah finds a neighbor to interpret. Flor admits she is unable to maintain these hours because she has a daughter; out of desperation to keep Flor as their nanny, Deborah invites Cristina to come stay with them, acting as interpreter for her mother. Deborah immediately becomes attached to Cristina when she first arrived due to her beauty and begins to treat her more like a daughter than Bernice, taking Cristina shopping, buying her gifts, and getting her hair done. To make Cristina feel more comfortable, John gives the children a small glass-collecting project in which they receive money and includes Cristina. She was given $640 for her glass collection, which Flor finds out. The two argue with Cristina as the interpreter, and Flor wants to leave because of the awkwardness it will create afterwards, but John coaxed her into staying. She begins to learn English so she can communicate better with the Claskys.
In the meantime, John opens a new restaurant, but falls into a temporary depression because of the stress of the business, and Deborah is having an affair, dressing provocatively and leaving only at nighttime. She enrolls Cristina into a private school with Bernice, upsetting Flor, who wants Cristina to keep in touch with her Mexican roots and working-class values. She feels Deborah is overstepping her bounds and voices her objection to John, who tells her he is just as stressed because Bernice has no support system from her own mother.
Summer is over and it is Cristina's and Bernice's first day of school; that afternoon, Cristina was allowed to bring friends back to the Clasky's home from school, but not Bernice. Deborah tries to cover for Cristina to keep her there, but an angry Flor marches to her place. The now-sober Evelyn realizes that her daughter is having an affair and that her marriage is in trouble. She pleads with Deborah to end the affair, telling her she will never get another man as good as John. Deborah confesses to John that she cheated on him and begs him to stay so the two could talk everything over; however, John walks out and bumps into Flor, who was trying to tell him that she is quitting. He offers to give Flor a ride in his car, but he wanted to "hang out" when they arrived at her bus stop, and they ended up going to his restaurant, where he cooks for Flor. They become extremely close to the point of falling in love, but Flor is afraid of the consequences of having an affair while they both have kids. While the two enjoyed the "conversation of their lives", Flor tells John she loves him, but leaves him before he could kiss her. Deborah, though, becomes desperate and seeks to call John, but Evelyn prevents her from doing so. She blames Evelyn's mistakes as a mother that led Deborah to cheating on her husband; the two become closer as mother and daughter.
Flor quits and takes her daughter home, upsetting Cristina, who got along well with the Claskys. On their way home, she tells Cristina that she cannot go to the private school anymore, upsetting Cristina even more; she screams in the middle of the street, accusing Flor of ruining her life. Flor loses patience with Cristina after she asks her mother for space, similar to the Claskys asking for space when they cannot solve a problem. Flor explains to her daughter that she must answer the most important question of her life, at a very young age: "Is what you want for yourself to become someone very different than me?" Cristina considers this on their bus ride home, and they make up and embrace.
Cristina, on her paper in applying to Princeton University, acknowledges that her life rests firmly and happily on the simple fact that she is her mother's daughter.

Question: Why is Christina upset with her mother at the end? 
Answer: She got along with Claskys well and didn't want to leave them; she wanted to continue going to a private school
Alternative answer: Flor quit her job