^ Only when it's clear the parents are grossly incompetent in such matters. If their kid is already morbidly obese, and the 'rents won't do bupkis about it, then by all means.
However, let's not kid ourselves. The government's track record as parental substitutes is ABYSMAL at best. I've already seen how well the government is doing on the drug war (they're not). On this issue, frankly, the nanny staters can go jump off a ledge.
Nothing of value would be lost if they did.