We examined the pathway from grandparental childcare support at age 3 to child social and emotional outcomes at age 7 through maternal mental health and mother-child relationship at age 3 in a sample of n = 1495 biological mothers and their children from the UK's Millennium Cohort Study. Structural equation modelling showed that time spent in the care of grandparents at age 3 was not associated with the child's social and emotional outcomes at age 7 via maternal psychological distress or mother-child conflict and mother-child closeness. Maternal psychological distress at age 3 was associated with poor child social and emotional outcomes at age 7 both directly and indirectly via mother-child conflict and mother-child closeness. In the early years, grandparental childcare, as primary form of childcare arrangement, may not be significantly associated with maternal mental wellbeing or parenting capacity, which are both beneficial for child socioemotional development.