This paper examines two issues concerning nonagreeing "don't" in child English, e.g., "He don't fit". (1) Do children know that "don't" consists of auxiliary "do" plus sentential negation, or do they misanalyze it simply as negation? I argue that the former claim yields both empirical (distributional) and conceptual advantages, while the latter does not explain what it was designed to explain. (2) If it is not misanalyzed, why does this form fail to agree? I consider two accounts that assume it is part of the Root Infinitive stage--one based on a misset parameter involving how agreement is spelled out (Guasti & Rizzi 2002), and the other based on underspecification of Infl features in syntax (my alternative proposal)--and explore their divergent predictions. I argue that the underspecification approach requires fewer stipulations about how children differ from adults, particularly for capturing "do"-omissions in "medial neg" environments. (Contains 43 footnotes and 17 tables.)