In this poster, I analyze the association between family complexity and the well-being of children. I focus specifically on the developmental and social well-being of children, analyzing extracurricular participation and school outcomes. I use data from Wave 1 of the 2014 panel of the Survey of Income and Program Participation (SIPP). The SIPP is the only nationally representative dataset that allows for complex measures of family structure that encompass sibling composition and coresident parents’ fertility histories. I find that complex sibling arrangements are not uncommon in families today. However, children living with fewer siblings, only biological siblings, or with two parents had higher extracurricular involvement and more positive school experiences. Controlling for race and poverty narrowed this gap, but did not erase this relationship.