You may find your black and white ideological extremism comforting, but in the real world, where real people live, collisions of liberties means there are no absolutes. In general terms, your freedom of action ends at the tip of my nose, so your liberties are not absolute.
Children have the same fundamental liberties as their parents, but are not deemed to have the emotional or cognitive maturity to exercise those liberties responsibly. The child's guardians is thus given considerable legal and moral authority over the child, but that authority is not absolute, because to make it absolute would essentially render the child's liberties null and void. And thus the courts can force a child to have life-saving procedure like a blood transfusion despite the protestation's of the child's guardian.