This news isn't just important for the here and now; it holds big implications for the future. Overweight or obese preschoolers are five times as likely as other kids to have lifelong weight problems, increasing their risk of heart disease, diabetes and early death.
The researchers at the Centers for Disease Control and Prevention who found the positive trend among poor children can't put their finger on why it's happening, though they do suspect that various government efforts to publicize the child obesity problem are making a difference.
They specifically cite improvements in the nutritional content of food packages provided by the federal food program for low-income women and children. The children in the obesity study were part of this program.
What's more, kids today drink less sugary soda than they did in 1999 (8 percent less, a recent study shows); breastfeeding, which has been correlated with relatively healthy weight gain in babies is gradually becoming more prevalent. In 2009, almost 79 percent of infants at least started breastfeeding.
How can we build on this early success? Many states and cities have banned sugar-sweetened drinks in K-12 school programs. Preschools and day-care centers should be coaxed to follow suit - and to restrict not only sodas but also fruit drinks and sports beverages. Water can do the trick.
It's also true that the challenges of weight control restricting sugary drinks, limiting TV time, serving plenty of fresh vegetables and fruit at meals fall disproportionately on poorer families. It's important to direct healthy-weight messages to children in this cohort.
Small children of every income level are still vulnerable to advertising of fast food and soft drinks on television. From 2009 to 2011, TV food ads targeted to children ages 2 to 5 increased by 8.3 percent, the Federal Trade Commission has found. If the food industry can't, on its own, restrict commercials for children to foods that meet the Dietary Guidelines for Americans, then local, state and federal policy makers should consider setting mandatory standards.
As the United States makes some strides against obesity in small children, the rest of the world will want to pay attention. The World Health Organization estimates that 40 million kids younger than 5 worldwide are now overweight, including 10 million in developing countries, adding to an increasing global burden of noncommunicable diseases.
And to be clear: Obesity still afflicts one in eight American preschoolers and one in six older children and adolescents. The news that the rate is falling among poor children is good, but it's only a start.