Millennial parents dream of a golden age gap between kids. It comes with a catch.

Getty Images After Kristin Darnell had her first child at 34, when her husband was 40, the plan was to try for their second in two years. But when the time came, they weren't ready. One of the biggest concerns was the cost of childcare. Having two kids in day care at once would have been astronomically expensive, even though Darnell works in ingredient sales for the food and beverage industry, and her husband is an engineer. The couple was eventually able to welcome their second child a little…