If life is about happiness (and it is, both now and in eternity), this poll speaks volume to the state of American life. From The Atlantic Monthly:
Today, as in 1972, roughly 50 percent of Americans describe themselves as “pretty happy,” just over 30 percent describe themselves as “very happy,” and around 15 percent say that they're “not too happy.” Nor have other trends in happiness changed much over the last three decades. Money bought happiness in 1972 at roughly the same rate it does now: rich people are more than twice as likely as people in the lowest income quartile to consider themselves “very happy,” while those with incomes in the middle fall somewhere in between. (The fact that the rich are consistently happier than the poor, but that neither group has grown appreciably happier overall despite three decades of rising per capita income, suggests that being richer than your neighbor, rather than being well-off in absolute terms, makes all the difference.)
That last parenthetical is significant. Money doesn't make you happy and the lack of it doesn't make you sad, unless you exist based on the situation of others. If you look at yourself without regard to others, you're less likely to feel want or (worse) pride.
In an age of mass media, this is hard to do. TV and film constantly show people who live materially better than the average American. Surely this leads to an increase level of discontentment. Increased mobility does the same thing. Your child would probably be happy with his handful of belongings, but when he comes into contact with kids whose parents provide luxuriously, your child instinctively notices his lack and might be saddened.
What's the answer? I don't know, but I'd limit children's access to rich kids and TV, for starters. But more importantly, they need to learn the virtue of spiritual poverty: detachment from the things of the world.
And most importantly, we need to cultivate detachment ourselves. If your children see that you're happy with less, they're more likely to be happy with less as well. If you grouse about your lack of money and possessions, your children are more likely to grouse, too.