I know of a child just turned 5years old and weighs 90 lbs estimated he has dicolored toenails is this a cause?

Possibly: If you mean, because of obesity. Excessive weight can put excessive stress on feet in shoes and affect toenails, either by trauma or fungus infection. However, a poddiatrist should be seen to evaluate.

There are : Many medical conditions that can cause changes in the nail unit. Some affect the color, some the integrity. Diabetes has been documented to cause color change in toenails. However, because so many other things can cause these changes i don't think you can prove it is because of the diabetes. And not all diabetics get color changes in nails. In fact, likely very few do.