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Never mind, found it. It’s called Snapshots by Norma Klein, and it was written in 1984.
The summary reads as follows:
Thirteen-year-old Sean and his friend Marc find themselves in serious trouble when some photographs they take of Marc's eight-year-old sister are interpreted by the district attorney's office to be pornographic. Sean is already confused by his upcoming bar mitzvah and by his conflicting feelings about girls, so the uproar over the pictures just adds to his confusion.
Summary taken from Committee on the Junior High and Middle School Booklist on the National Council of the Teachers of English (James E. Davis and Hazel K. Davis, editors), Your Reading: A Booklist for Junior High and Middle School Students (7th ed. 1988) at 47.

This YA book was written in the early 1990s or before. It’s written in first person, about an eighth grader named Sean. One day, while at a friend’s house, Sean and his friend were taking pictures of his friend’s little sister (I think she was wearing a bathing suit), and she was doing silly things, including kissing a balloon. When the pictures get developed, someone at the photo lab suspects child pornography and raises the alarm. As a result, Sean’s and his friend’s parents have to hire a lawyer.