Six months ago the Atlantic Theater presented Kimberly Akimbo, the musical tale of a 15-year-old girl whose young mind is trapped in a quickly aging body. Now, with the premier of The Bedwetter, it offers up the story of Sarah (Zoe Glick), a 10-year-old girl with a troubled, adult mind trapped in a child’s body that is always letting her down. And though Kimberly is headed toward an early death while Sarah advances toward certain fame, it is the latter character who wants our sympathy. She struggles, however, to fully earn it. In this uneven production, as director Anne Kauffman has discovered, a depressed kid with a foul mouth makes for a problematic protagonist.