I know that the title of this article is probably rather intriguing, so let me explain myself from the get-go. The middle-aged millionaires who may care for you in the future are your children. They may not be millionaires now, but you can help them become millionaires.

Let's assume that you're in your 30s or 40s and that you have two children, both around age 10. As they become teens (or earlier, if they're clever and precocious), you can encourage them to spend time in our Teens & Their Money nook, where they'll learn how and why to earn more and invest. Alternatively, consider giving them a copy of our well-regarded book (check out its reviews on Amazon.com), The Motley Fool Investment Guide for Teens: 8 Steps to Having More Money Than Your Parents Ever Dreamed Of.

But why wait for them to get a little older and act on their own, when you can jump-start things for them? After all, they have the most of something that's critical for most investing: time. And the sooner they start, the more their money can grow.

Running some numbers
Let's say that you're aiming for them to have $2 million by the time they're 50. (If they end up spending just 3% of that on you each year, that would be a generous $60,000. If there are two of them, that would come to $120,000.)

Here's how their initial investment would grow at several average annual rates over 40 years (age 10 to 50):

At It would increase
9% 31-fold
10% 45-fold
11% 65-fold
12% 93-fold
13% 132-fold

So to get to $2 million at 9%, you'd have to start them off with around $64,500. With 10% growth, it would be $44,400. With 13% growth, just $15,200.

Keep in mind
Of course, no one knows what the average annual growth rates will be for the overall stock market or other investment options over the next 40 years. The best option is to be conservative and have low expectations -- and hope for better.

But the above examples do show how powerfully money can grow over long periods -- so much so that you could invest sums today that are so small that you might reasonably forget about them for 40 years, letting them sweetly surprise you down the road. Say you socked away just $15,200 for your kid today and instead of growing at 13% annually (on average), it grew at 9%. It would still come to nearly $500,000 -- not an inconsequential sum, considering the little it grew from.

Also, remember that these examples revolve around a single initial investment amount. Keep contributing over the years and the ending sum can be much, much higher -- several times higher, in fact. That $2 million could become $4 million or more, enough to park you in a snazzy retirement home, don't you think? (A less selfish way to think about it is, of course, that your children will likely end up with less financial stress, knowing that they've got a sizable and growing cushion for themselves and their future.)

How to earn it
So how, exactly, should you invest these early sums, in order to get those 9% or 10% or 13% returns? Well, the simplest option for long-term money is a broad-market index fund, such as one based on the S&P 500. Plunk your kids' money into that, and instantly, some 2% of it will be invested in Microsoft (NASDAQ:MSFT), 2% in Citigroup (NYSE:C), 1.6% in Pfizer (NYSE:PFE), 1.1% in Cisco Systems (NASDAQ:CSCO), 1% in Intel (NASDAQ:INTC), 0.85% in Verizon (NYSE:VZ), and 0.75% in Home Depot (NYSE:HD), among 493 other firms. That kind of investment has averaged 10% annual growth over most of the past decade.

To do better, seek out some exceptionally healthy, growing companies -- or let some smart mutual fund managers do so for you. If you're in the market for a few market-beating funds with low turnover rates (not to mention great managers, low fees, and top-notch track records), we'd love to introduce you to some via our Champion Funds newsletter. It's up some 17%, vs. 9% for its picks' corresponding indexes. (Try the newsletter for free and you'll be able to see all the funds analyst Shannon Zimmerman has recommended and how well they've done.)

Learn much more in these Zimmerman articles:

Microsoft, Intel, Home Depot, and Pfizer are all Motley Fool Inside Value selections.

Selena Maranjian 's favorite discussion boards include Book Club , The Eclectic Library, Television Banter and Card & Board Games. She owns shares of Microsoft, Pfizer, and Home Depot. For more about Selena, viewher bio and her profile. You might also be interested in these books she has written or co-written:The Motley Fool Money GuideandThe Motley Fool Investment Guide for Teens. The Motley Fool is Fools writing for Fools.