Information on children's writing in online spaces is scarce. What young writers know or need to know to be effective communicators online can inform elementary writing instruction and technology integration in writing classrooms. This study adds to the nascent research on children's online writing by reporting on the writing processes of eight second-grade children ("tweeters") when composing short-form writing online for their class Twitter account. This multicase study, with data drawn from think-aloud transcripts, video-stimulated recall interview transcripts, and published tweets, suggests that young children's short-form writing includes a motivation to tweet, goal setting, and writing schema knowledge specific to Twitter. On the other hand, children's short-form online writing for Twitter shared attributes with traditional writing typically emphasized in school. We discuss how writing for social media can enhance children's developing writing processes. Implications for elementary writing instruction and social media integration in similar classroom contexts are discussed.