This study investigated the relationship between grandparental investment and child outcomes using data that were obtained from three waves of a longitudinal British Millennium Cohort Study that included children between the ages of 9 months and 5 years (n = 25,446 person-observations from 14,065 unique individuals). Grandparental investment was measured by parent-grandparent contact frequency and grandparental financial support. Child cognitive development was measured using the British Ability Scale and emotional and behavioral problems are measured using the Strength and Difficulties Questionnaire. The results showed that grandparental investment is associated with increased cognitive assessment and decreased problems among children. However, these associations occurred because of between-person effects and did not hold for within-person analyses that compare the same participants over time. Therefore, the results did not provide evidence for a causal association between grandparental investment and child outcomes.