Rosa Feddersen and her husband bought their dream retirement home on a lake in Oklahoma City five years ago. He, a pilot for U.S. Airways, was nearing the end of his career, and the area had everything the couple wanted.

But when they learned their first grandchild was on the way in 2009, their agenda changed.

After pleas from their daughter, they moved back to Pennsylvania to help with the baby. Their daughter and son-in-law are both surgeons, and Feddersen sometimes watches her granddaughter, Nora, 70 hours a week. While it's a lot of work, she says, the arrangement seems to be working for everyone.

One reason: When it comes to taking care of baby, parents and grandparents try to stay out of each other's way.

"When I'm watching her, they pretty much understand that what I say goes," Feddersen says. "But when they're home, I totally back off."

That kind of mutual trust is essential to a successful child care arrangement with grandparents, says Lawrence Balter, a child psychologist and parenting expert who is also a professor emeritus at New York University.

Sharing child rearing duties is almost never simple.

"Both generations are going to have their ideal way of doing things," he says. "You have to be able to navigate and find a happy medium."

More and more families are finding themselves in these murky waters. According to the most recent Census data, 30 percent of preschool children with employed mothers are cared for by a grandparent, while 21 percent attend a day-care center. And the economic woes of the past few years have led parents to seek more help from relatives, says Donna Butts, executive director of Generations United, a nonprofit based in Washington.

In addition to being a money-saving option — the average cost of center-based day care is approaching $12,000 a year — letting grandparents take care of the kids has other benefits, Butts says. Children learn about their family history and are cared for by adults who love them, while parents can have more flexible schedules. As for the grandparents, a 2007 study by Linda Waite, a professor of sociology at the University of Chicago, found that grandmothers who baby-sit 200 to 500 hours per year exercise more and get depressed less often.

But these arrangements can also be tricky because there isn't the same clearly defined code of conduct that would apply with a professional day-care provider. Balter shares these tips for ensuring that everyone remains healthy and happy.

Set clear expectations. Determine how many hours each week the grandparent will care for the child, during what times and at whose house, and do your best to stick to the plan. Also, if there's compensation involved, decide on the amount in advance. Is the grandparent expected to do any chores during the day? Make sure everyone agrees.

Establish routines. Work together to create a rough schedule for the children's day, including nap times and meals. This is a good opportunity for mom or dad to fold in more detailed requests — for instance, if there are certain foods they do or do not want the child to eat.

Don't be critical. Remember you're on the same team. Instead of a parent saying, "My daughter doesn't go to bed because you're getting her overexcited after dinner," try phrasing it without accusation: "Let's run through the schedule and see what we can do so she'll be calmer at night." This advice applies to grandparents, too. If you notice the parents doing something ineffective, instead of correcting them, try offering gentle suggestions, such as, "When you were a kid, we did it this way and it seemed to work."