What was in the two letters Claudia mailed when she and Jamie got off the train?

One letter is to their parents telling them they have run away and the other is to mail in Box Tops.

Claudia and Jamie run away from home and go live in a museum. That is pretty unusual, but Claudia is unusual. She is leaving because she is the oldest and feels unappreciated. She brings her little brother Jamie because he is quiet and has saved his money.

Claudia is twelve and Jamie is nine, so they are going to be missed. They are going to the museum by train, and Claudia stops to mail a letter. She is hoping to buy them some time. Jamie asks her what was in the two letters. She tells him,

"One was a note to Mom and Dad to tell them that we are leaving home and not to call the FBI. They'll get it tomorrow or the day after." 

"And the other?" 

"The other was two box tops from corn flakes. They send you twenty-five cents if you mail them two box tops with stars on the tops. For milk money, it said" (Chapter 2).

The two children are looking for independence and adventure. While Claudia is smart, her two letters show she has sort of strange priorities and is incredibly naive, even for a twelve-year-old. There is no way the FBI will not look for them, and expecting her parents no to take action for the two days until the letter arrives shows Claudia doesn't understand how her parents will react to losing two of their children. In addition, twenty-five cents is not enough to fund a runaway, and where would they send it to? (Jamie points out the folly of the Box Tops.)

Claudia chose the Metropolitan Museum of Art because it is quiet, orderly, and has plenty of interesting stuff to look at. She desired "a large place, a comfortable place, an indoor place, and preferably a beautiful place." The museum fit the bill.