Children exposed to socioeconomic adversity have elevated levels of psychological difficulties immediately and long-term. However, few studies have examined the consequences of long-term patterns of dynamic trajectories of family income. The Danish National Birth Cohort (DNBC) is a longitudinal, population-based birth cohort study (1996–2002). Data on household poverty from the year before birth until the child was 10 years of age (n = 12 measures) were obtained from the National Danish Registries and modeled using semiparametric group-based modeling. Child mental health symptoms were measured at 11 years using mother and child-reported Strengths and Difficulties Questionnaires (n = 40 192), and the child-reported Stress in Childhood (SiC) scale (n = 46 284). Four categories of family socioeconomic position were identified: 1) No poverty (83.5%); 2) Intermittent poverty, representing families who alternate between being above and below the poverty cut-off (8.6%); 3) Poverty during the perinatal period (4.9%); and 4) Chronic poverty (3.0%). Controlling for several early life characteristics of the family, mother and child, intermittent poverty vs. no poverty was consistently associated with child psychological difficulties (any problem: RR = 1.38, 95% CI: 1.16–1.64; conduct problems: RR = 1.38, 95% CI: 1.16–1.64; and stress: RR = 1.07, 95% CI: 1.02–1.12). An association was also found between perinatal poverty and children's symptoms of hyperactivity/inattention (RR = 1.28, 95% CI = 1.03; 1.59). We found no associations between chronic poverty and any of the outcome measures when adjusting for early life risk factors. Children growing up in households characterized by financial instability have elevated levels of psychosocial symptoms, especially externalizing behaviors, as well as stress in early adolescence.