Studies suggest that welfare-to-work programmes increase lone mother's employment, but their impact on child and adolescent socioemotional development is unclear. The lone parent obligation (LPO) reform introduced a requirement for lone mothers entitled to unconditional Income Support (IS) to seek paid work actively as a condition to receive benefits. We use data from the UK Millennium Cohort Study to examine the impact of work search requirements for lone parents on child and adolescent socioemotional development. We apply a difference-in-differences approach that exploits gradual changes from 2008 to 2017 in children's maximum eligibility age to receive IS and assess effects on child and adolescent Strengths and Difficulties Questionnaire (SDQ) scores. We find that the LPO reform led to a ten-percentage point increase in lone mothers' employment but no change in the risk of family poverty. The reform led to a small but statistically significant increase in adolescents' SDQ scores, which indicated worse mental health and was largely driven by increased emotional problems. It also led to an increase in mothers' distress and poor self-rated health, and mothers' reports that time spent with children was insufficient. Our findings highlight the need to consider trade-offs between employment gains and child and adolescent socioemotional development in assessing the net impact of welfare-to-work policies targeting lone parents. •The Lone Parent Obligation reform led to increases in lone mothers' employment.•There was no significant change in the risks of family poverty.•Lone mothers experienced higher risks of psychological distress.•The reform did not improve adolescents' socioemotional development.