59% of well-off millennials say they'd make 1 big retirement mistake for the sake of their kids

Would you keep working longer than you want to, or under-fund your own retirement, if it meant your children wouldn't have to take on student loan debt? A lot of millennials would be willing to make the sacrifice.

That's according to investment company Ameriprise, which surveyed 453 older millennials, classified here as those ages 30 to 37, who had at least $100,000 in investable assets. While this was not a large sample size and respondents are wealthier than average, other studies that considered millennials and their children's college costs have found similar results.

More than any other generation, milllennials want to help their kids pay for college. Only about 41% of Gen X (ages 38 to 53) and 21% of baby boomers (ages 54 to 69) with $100,000 to invest said they delayed or would be willing to postpone their own retirement to pay for their children's higher education.

Financial experts urge caution. While you may have good intentions when you plan to help your kids afford college, they say, doing so at the expense of your retirement is a mistake.

"You're not a bad parent if you're not helping fund your kids' education," Charles Sachs, a Miami-based financial planner, tells CNBC Make It. If you have got extra money to put away for their education, Sachs says, great, but it's not wise to risk your future to prioritize your kids' college costs.

For many millennials, helping their children with college is an emotional decision, says Doug Boneparth, a financial planner and a member of CNBC's Advisor Council. Many of them have had to shoulder student loan debt themselves and would like to alleviate that burden for their kids.

"There's an emotional side to it," Boneparth says, that millennials "don't want their children to go through" having to pay back loans. But you want to avoid making decisions based on your feelings, he says. Once your emotions are involved, "that's typically when solid financial thinking goes by the wayside."

Opting to under-fund your retirement in favor of saving for your kids college and planning to work indefinitely to make up the deficit are just not practical, experts say. Especially when you consider that a majority of Americans say they already plan to need to work long after the age of 65 "out of financial necessity," according to Pew Charitable Trusts.

For many people, it just may not be feasible to put off retirement. In a 2015 survey fielded by Voya Financial, 60% of retirees say they had to retire before they planned to. About 16% said the unexpected change was due to health reasons.

"We have no idea what college will even look like in 15 to 20 years, but we know with a fair amount of certainty that [millennials] will either not want to work or not be able to work one day," says Kaleb Paddock, a financial planner based in Parker, Colorado.

Focus on yourself first, says Mark Beaver, a financial advisor in Dublin, Ohio: "There's a reason on an airplane they say to put on your own oxygen mask before helping a child." Make sure you are on track with your emergency fund and putting money away for retirement before beginning a college savings plan.

And when it comes time to pay for college, your children will likely have options. You can use a number of different loans, scholarships and grants to finance college, says Las Vegas-based financial planner Luis Rosa.

"Consider the fact that there's pretty much financing available for most life decisions, except retirement," Rosa says.

If you're a millennial parent, experts say there are a few higher priorities you should deal with before you start putting money into your children's college funds.

First: Set up an emergency savings account. Most experts recommend that you have three-to-six months of living expenses set aside in an account earmarked for a crisis. If you really want to make your money work for you, keep it in a high-yield online savings account like Ally's, which is currently paying 2.2% APY.

You should "have some emergency savings to protect you from life's bumps," says Carol Fabbri, a financial advisor in Conifer, Colorado. "Whether it's a broken arm or a lost job, you need a cushion."

Next, try to max out your retirement accounts. "The best thing young parents can do to help pay for college in the future is to begin saving for retirement early," says David McPherson, a financial planner based in Falmouth, Massachusetts. That's because a parent who is well-situated financially can better help their kid when the time comes.

"Everyone comes out ahead if the parents start saving for retirement early," McPherson says.

If you still have money leftover after maxing out your retirement accounts, experts recommend putting it into a 529 college savings plan. These are tax-advantaged accounts specifically designed to help families save for education costs.

About 30 states offer tax deductions for those contributing to a 529. Plus, investments made in these accounts grow tax-free. Every state has a plan for residents and several states allow anyone to participate, even if you don't live there.

"If you aren't sure how to invest in a 529, many of the target-date options are a great choice," Beaver says. They will automatically adjust the investments based on the child's age so when they get to high school, there is less risk in the portfolio. "You wouldn't want a 529 to decline in value potentially right before they enroll in college," he says.

If you have kids, you should open a 529 right away — even if you're not investing money regularly. "If the children have at least 10 years until starting college, then 529 plans still make sense," says Minneapolis-based financial planner Laura Nickolay. The longer you wait to open one, the less it will pay off.

Keep in mind that other family members can also contribute to 529 plans. So even if you're juggling expenses and savings, grandparents may be in a position to help. Contributions can make meaningful holiday and birthday gifts.

It's also important to remember that working to pay for college is not your only chance to help your children financially, says Sean Pearson, a financial advisor with Ameriprise.

"If down the road you're on track with your goals and would like to help your child or even grandchild reach their goals, you have options," he says. Parents and grandparents can help repay student loans, provide a down payment on a home, contribute to wedding expenses or treat them to a vacation.

"While it's admirable to consider delaying saving for yourself to help your children, I tell my clients that it's imperative to balance the desire to help with the knowledge that under-funding retirement may create a difficult situation down the road," Pearson says.

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