Poverty is experienced by about 15% of children under the age of six in the United States at any given time and over 40% of children at least once prior to kindergarten entry. Given that many families experience dips in and out of poverty prior to school entry, increasing research focuses on early household economic conditions and family income instability among such families. Using Early Childhood Longitudinal Study-Birth Cohort data (2001–2007) sampling children in poor and moderate income families and two relevant theoretical frameworks, the present study investigated (a) the extent to which income varies around the official poverty threshold (as well as other relevant thresholds used to determine social safety net program eligibility) in a subsample of low-to-moderate income families, and (b) the early, time-variant factors associated with membership in specific birth to kindergarten income trajectories as well as fluctuations around well-used income thresholds in a subsample of low-to-moderate income families. Sixty-seven percent of children in the study were classified in the stable low income trajectory, and findings suggest the majority of these children oscillated above and below 100% Federal Poverty Level (FPL) at various times throughout early childhood (55%). Results also suggest birth, child, and family characteristics such as changes in maternal depression, maternal work status, and household composition differentiate income trajectory class membership and are associated with income fluctuation around 100% FPL and 130% FPL. Findings are discussed in the context of current dialogue about the nature of family poverty over time, and the limitations of social policy responses that attend to family needs at one point-in-time without consideration of trajectories and the consequences of cyclical poverty for children and their families.