It depends upon whether you can reasonably do your job with the cane and the state/condition of your knees. That you did you job with a cane before is evidence that you can, but does not by itself prove that: your knees may be worse now, for example, than they had been. If in taking mentally challenged adults around, helping with their daily activities, etc. you reasonably need to be able to move faster than you can, or support more weight, or have both hands free/available, then they could refuse to let you work: while employers do need to make "reasonable accommodations" for employee medical conditions or disabilities, the employee still needs to be able to do the core functions of his/her job. So if you can do the job, then not letting your work may well be illegal discrimination-based employment discrimination, and you could file a complaint with the federal EEOC or your state's equal/civil rights agency. But if you can't do the job, then this would be legal.