You don't have to hold stocks for that long to get a nice bang for your buck. However, focusing on long-term investing cuts down on costly commission charges that eat away at portfolios. If you are thinking about investments for a child, the only investments you should be considering are stocks. Because your child has time on his or her side. And with a lot of time comes the ability to handle a lot of investment risk.
Volatility is a measure of short-term risk.
Who knows how investments will behave today, tomorrow, a week from now, a month from now, a year from now. The shorter the time frame, the more random the likely returns. Buying and holding stocks for one year is risky. When you lengthen holding periods, however, that spread reduces. The last 30-year period in which bonds beat stocks ended in 1861, at the onset of the U.S. Civil War.