One child one vote!
Q. What does a child need besides being born and grow up in a loving family?
A. To have a voice of his own and his particular needs represented!
Q. Why do we not recognize the right to vote of ALL citizens, regardless of age?
US legislatures and the courts could assure the recognition of the right to vote of all citizens, regardless of age.
The citizens under the age of 18, as with all other rights they make use of as minors, would in such case be represented at the voting booth by their parents or whoever holds their legal guardianship.
Those accountable for how they represent their child at the voting booths would be made more aware of their responsibilities; and the importance of family.