Answer:
This issue depends largely on the laws in place where the child lives. In the United States, children can be copyright holders and are entitled to license their works in the same manner as adults. However, they may have the right to disaffirm certain types of legal agreements, including licenses. In many parts of the United States, for example, children have the ability to disaffirm some types of agreements under certain circumstances once they reach the age where they are considered adults within the relevant jurisdiction. We are unaware of any attempt by a licensor to exercise the disaffirmation right with respect to a CC license applied to a work.