Is it permissible for a widowed mother, who has custody of her child, to travel with the child for study purposes, given that she has a court order of guardianship?

A woman's travel with her child is permissible on condition that there is a mature, sane mahram present. As for a young child, he is not fit to be a mahram due to his immaturity. Likewise, it is not permissible for a woman to travel while she is in her iddah (waiting period) due to the death of her husband, except out of necessity.