Long before a child enters a classroom, their future is shaped by the emotional well-being of the person who raises them. Through a mixed-methods study of 564 caregiver-child pairs and 42 in-depth interviews in rural Zhejiang, one of China's wealthiest provinces, this research reveals a surprising reality: despite economic prosperity, developmental delays remain widespread, and nearly one in six caregivers experiences symptoms of depression, anxiety, or stress. Poor caregiver mental health is associated with worse child language and socio-emotional development, while interviews uncover an often-invisible cycle driven by caregiving burdens, limited mental health awareness, and insufficient support. By challenging the assumption that affluence alone protects families, this study highlights caregiver mental health as a critical yet overlooked foundation of early childhood development and calls for policies that support both children and the adults raising them.