وعليكم السلام ورحمة الله وبركاته To make someone as a Mahram guardian either during traveling or any thing else , he must have reached the age of puberty. Because the Mahramiyyah (guardianship) is a kind of responsibility , and responsibilities in Islam are not obligatory upon a person who has not yet reached puberty. As for enjoining upon them some of the obligations and responsibilities before they reach their puberty , the intent behind that is only their education, disciplining and nurturing them upon goodness . But to make these things obligatory upon them , then it is a must that they should have reached their puberty . This is the conclusion in this issue based on the general and specific evidences related to the mater . والله أعلم