For some parents, the choice between working or staying home to take care of their kids comes down to a simple equation: Can I make more than I spend on child care?

And sometimes the math just doesn't add up. Among millennials (ages 24 to 39), 38% say their primary child care is one parent staying home with the kids, according to a recent CNBC Make It survey conducted by YouGov of nearly 4,000 U.S. adults, almost 400 of which were millennial parents with at least one child under the age of 10.