Families with young children tend to have lower incomes than other families. The current child tax credit provides substantial benefits to families with children, but the maximum benefit of $1,000 is the same for all children under 17, regardless of age. We analyze a proposal that would provide an additional credit of up to $1,000 per child under 5. An income boost during these critical developmental years could provide lifelong benefits. The credit would deliver about $18 billion in benefits in 2017 and would be refundable, starting with the first dollar of earnings, so that even very low–income families could benefit.