One out of every three children under the age of 5 in developing countries lives in an environment that impedes human capital development. Children growing up in resource-poor settings are at an increased risk of early developmental delay due to risk factors such as being in environments that lack cognitive stimulation, nutrition, or care in the home environment. Given that early developmental deficits are difficult to reverse later in life, such deficits are key drivers of inequality and impediments for intergenerational mobility. This policy brief reviews this problem and then proposes a design of cost-effective, scalable, and sustainable parental training programs (focusing on psychosocial stimulation for young children) as a global strategy to improve the developmental opportunities of children before they reach the age of 5 in developing countries, to remediate social inequalities, and to boost long-term economic development. We present concrete policy recommendations for the implementation of such programs at scale.