<p>This article evaluates the link between family instability and children’s socioemotional development. Using a longitudinal survey, we can track the number and role of household members and measure the children’s development. We analyze the entries of younger siblings and the exits of grandparents and fathers during the first ten years of a child’s life, in relation to two benchmarks: stable households (no changes in household composition) and nuclear stable households (mother, father, and child in every period). The entry of a younger sibling affects children’s externalizing and internalizing problems, particularly in households with lower educational levels. These results relate to the changes in the mother’s time use and her inability to substitute market work with housework, leading to an increased overall workload. The father’s exit from the household is associated with greater behavioral problems in children in relation to the nuclear stable household benchmark, but not observed in households with other structures. Also, the father’s permanent absence has a significant association with children’s externalizing and internalizing problems, regardless of the mother’s educational level. Regarding grandparents, their permanent presence increases children’s externalizing problems when the mother has a lower educational level. Taken together, and considering that development levels were already lower for children with low-educated mothers in the first wave, these results highlight the diverging consequences of family instability in children of low and high-educated mothers, widening the initial development inequalities.</p>