Is it permissible for a woman to pray in a jilbab or wide trousers if her legs (but not her feet) become uncovered during prostration, especially if the garment is not long and does not reach the ground?

There is no difference between "rijlain" (legs) and "qadamain" (feet); they are one and the same. The feet of a woman are 'awrah according to the majority of scholars. It is apparent that her feet fall under the category of minor 'awrah, the sight of which is not immodest. Therefore, if a small portion of the 'awrah is uncovered unintentionally, while she is not negligent in covering it, then the prayer is valid.