Is it true that overweight children are likely to experience puberty later than their underweight peers?

Quite the opposite. Children who are overweight and and or obese are often early bloomers and go through puberty earlier than their normal weight peers. Some of the signals that lead to the start of puberty are tied to body weight, in particular a body weight or amount of body fat that signals the brain that the child is of an age when puberty should start. Some, but not all children who are underweight will have delayed growth and puberty.
Overweight. Just the opposite-- overweight girls are more likely to have precocious puberty. Check out the olympic gymnasts -- not so developed due to keeping there lean body mass down as well as activity delaying puberty. No correlation in boys to weight.
No !! Earlier. No. Overweight/obese kids go through puberty earlier (sometimes much earlier) than their normal weight peers. Underweight kids may have delayed puberty. Early puberty puts kids at higher risk for short stature, gyn cancers (girls), prostate cancer (boys), and the overweight/obesity puts them at higher risk for heart disease, high blood pressure, diabetes and stroke.