All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The due condition for the validity of the prayer of a woman is covering her whole body except for her hands and face. If she wears a Jilbaab (Islamic robe or long, loose gown) that covers her whole body, except for the hands and face, it will be enough for her and she is not obliged to wear trousers or knee-length shorts beneath it. However, if the Jilbaab does not cover the tops of her feet, for example, she is obliged to cover them with trousers or the like. It is recommended to wear a Jilbaab or gown over the trousers while performing prayer, since performing it in trousers (without wearing anything over it) is disliked. Imaam Ash-Shaafi‘i said in his book, Al-Umm:
"It is recommended for women not to perform prayer in a shirt except if she wears an Izar (cloth covering from the navel to the knees or a wrapper), trousers (or knee-length shorts) beneath it or a gown over it. If a worshipper performs the prayer in a single shirt or garment that outlines his shape but is not transparent, his prayer is disliked. However, he is not obliged to perform it again. This applies to women for a greater reason. If she performs the prayer in a veil and tight-fitting clothes that outline her body, it is recommended for her to wear a loose gown or Jilbaab over it so as not to outline her body."
Allaah Knows best.