Would you forcibly hold down a 12 year old to give them a vaccine despite them refusing and resisting?
That can and does happen. Do you think that children enjoy getting shots? Children generally do not have bodily autonomy, no. Parents can refuse certain non-critical medical care for their children, even if the child wants that care. The state can force a child to receive certain medical care, even if the child doesn’t want it. Whether it’s morally right or not to deny a minor bodily autonomy is a different question, but as a matter of law, they do not generally have bodily autonomy.