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 later, thanks to IUI, ending up with a three-and-a-half-year age gap between their children, who are now 6 and 3.

Last year, with both kids in day care, they spent about $37,000 for the year — a little over $3,000 a month.

Darnell estimated that they saved about $200 to $300 a month in childcare costs due to the wider age gap.

"Family planning's a tricky thing; you have one idea before you start and then sometimes that idea shifts," Darnell, now 40, told Business Insider.

This is the spacing dilemma, and Darnell's family isn't the only one facing it.

More than a quarter of moms in their 20s expect to have their next child in two to five years, a 2026 CDC report shows.

But that figure drops below 10% for moms in their 30s.

Waiting longer between kids can help spread out day care costs and mean only having one child in diapers at a time.

But as millennials are having kids later , their hopes for a "golden" age gap can collide with another clock: declining fertility.

If parents want more breathing room between babies, waiting can come with trade-offs and benefits.