Tiny infinities / by J.H. Diehl.

Enhanced descriptions from Syndetics:

When Alice's dad moves out, leaving her with her troubled mother, she does the only thing that feels right: she retreats to her family's old Renaissance tent in the backyard, determined to live there until her dad comes home. In an attempt to keep at least one part of her summer from changing, Alice focuses on her quest to swim freestyle fast enough to get on her swim team's record board. But summers contain multitudes, and soon Alice meets an odd new friend, Harriet, whose obsession with the school's science fair is equal only to her conviction that Alice's best stroke is backstroke, not freestyle. Most unexpected of all is an unusual babysitting charge, Piper, who is mute--until Alice hears her speak. A funny and honest middle-grade novel, this sharply observed depiction of family, friendship, and Alice's determination to prove herself--as a babysitter, as a friend, as a daughter, as a person--rings loud and true.