A primary goal of children's savings accounts (CSAs) is to provide children, especially in low-income families, a strong economic footing. The ability to do that, however, depends on how CSAs are designed and how much families contribute. This study uses projections from the Urban Institute's DYNASIM model to estimate the wealth building impact of CSAs under alternative scenarios that vary the design features. The results highlight three points relevant for any asset-building proposal or program: incentives make a difference, targeting can be difficult, and nontaxability matters for all subsidies.