The current study examined the association between low-income mothers’ and fathers’ cognitive stimulation during early childhood and children’s vocabulary knowledge in middle childhood and whether children’s vocabulary knowledge in early childhood influences these relationships. We tested five hypotheses: that parental cognitive stimulation during early childhood will have a direct association on child vocabulary knowledge at age 9, that the longitudinal association between cognitive stimulation from one parent in early childhood and child vocabulary knowledge at age 9 will be stronger when the other parent also provides higher levels of cognitive stimulation in early childhood (accumulated advantage hypothesis), that higher level cognitive stimulation of one parent will compensate for lower level cognitive stimulation from the other parent (compensatory hypothesis), that the longitudinal association between cognitive stimulation from each parent at age 3 and child vocabulary knowledge at age 9 will be stronger when the child also has higher vocabulary knowledge at age 3, that parental cognitive stimulation at age 5 will mediate the association between child vocabulary knowledge at ages 3 and 9. Using data from the Fragile Families and Child Wellbeing study (n = 1850), we found a positive and significant association between fathers’ cognitive stimulation at age 3 and child vocabulary knowledge at age 9. We did not find support for the other hypotheses. Implications for future research and interventions are discussed.