On paper, the Peabody Essex Museum’s “PlayTime” looks like an innocent exhibition of fun contemporary art for the whole family to enjoy—ideal for this Salem, Massachusetts, institution known for its allergy to pretention and its enthusiastic outreach to as wide a community as possible. And curator Trevor Smith’s dramatic opening gesture seemed to fulfill that promise: The museum’s oldest, grandest gallery is occupied entirely by Lara Favaretto’s Instagram-ready Coppie Semplici, 2009, an installation of colorful car-wash brushes spinning in pairs. It seems to be a cheerful photo op until