Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 369
Question: Is it possible for my son who is nine years of age to be my mahram, since I travelled with him; Is there anything wrong in that?
Response: Regarding the mahram, the Scholars have mentioned that two conditions must be met:
1) The coming of age (puberty);
2) Mental awareness.
So, whoever has not yet attained puberty then it is not correct for him to be a mahram; and whoever is not yet mentally aware then it is not correct for him to be a mahram since the intent behind the [presence of a] mahram is protection of the wife and maintaining her and preventing any attack upon her. [Clearly] the young child is unable to undertake any of these [roles].
So I say, now the woman – as per the question – has reached her destination, so she is not to return to her husband until he comes and collects her and takes her [back] with him. Alternatively, she can travel with one of her mahrams (father, brother, etc.) – from amongst those who have attained puberty and are mentally aware. As for the young child who is nine years of age, then he cannot be her mahram.
