There are many pharmaceutical drugs that can cause weight gain as a side effect.
Examples include diabetes medication, antidepressants, antipsychotics, etc.
These drugs don’t cause a “willpower deficiency” – they alter the function of the body and brain, making it selectively store fat instead of burning it.
Children are becoming obese, diabetic and addicted to junk foods way before they’re old enough to make conscious decisions about these things.