Is a mother sinful if her disabled 10-year-old son goes for treatment in a mixed-gender swimming pool that includes unveiled women, given her fear of the impact of what he sees on his imagination until puberty, and the unavailability of another place for treatment?

There is no sin upon the mother for taking her ten-year-old son for treatment to a place where women wear trousers and shirts, because he is discerning and not desired [in a sexual sense]. His ruling with non-mahram (definition) women is like that of a mahram. The awrah of a woman in front of her mahrams is said to be between the navel and the knee, and trousers and a shirt do not reveal more than that. The fact that the son might be affected by what he sees of their bodies does not impact the ruling on the permissibility or prohibition of a discerning child's gaze. It is recommended to book an appointment at a time free of women, if possible.