If a child is neglected by their parents and are severely malnourished, there's never a question that the child should be removed from their parents and placed in foster care. It's obvious that their well-being and health is in imminent danger.
Now, what about a morbidly obese child? One of my family law paralegal friends is actually dealing with a case now where the parents are trying to get their child back. The kid, at the age of 8, weighs almost 250 pounds. It wasn't the child's weight that was the original issue as to why he was removed from his parents' care, but it is now an issue as to why the child's foster parents are fighting back. And CPS really is taking it seriously. The birth parents fulfilled all the original requirements to get their kid back (parenting classes, anger management, complying with probation) but in the last review, the child's health was brought up. And it looks like the kid is on a verge of having congestive heart failure. At the age of 8. I will say that her office is completely split down the middle on this issue. They are representing the birth parents and are arguing that the birth parents did everything that was required of them initially. To put up more barriers against family reunification is wrong at this point. The birth parents might be required to take a health class to see how they can help their kid lose weight, but honestly, after that, I'm not sure what else could be required.
Now, the foster parents want to keep the kid until the kid loses some weight, learns healthy habits, and the birth parents are willing to feed the kid healthy meals and make sure he gets adequate physical activity. But that's not a guarantee that the kid would maintain his weight once he's back with his birth parents.
Is the foster system (read - the government) have the right to decide this? If so, that means a hell of a lot of kids should be taken away by their parents. The system as it is right now is not equipped to handle this kind of issue. More pressing matters for the foster system is for the kids that have been abused in more severe forms. Is allowing your kid to become this heavy a form of child abuse/neglect?
After Googling this issue sounds like it is an issue that will become more common in years to come. "Opinion" piece by AMA supports the idea.