Is it permissible for a woman to save money without her husband's knowledge in order to secure her daughter's future, given that the tailoring workshop is owned by the wife, and her husband is merely an agent over it?

A wife is entrusted with her husband’s money, and it is not permissible for her to dispose of it without his permission, unless he withholds her rightful maintenance, in which case she may take what is customary. If he provides for you and your children appropriately, then you do not have the right to take anything from his money for saving or other purposes without his permission. As for your husband’s conduct regarding your money and his denial of your ownership of the workshop, this is a matter of dispute that the court will decide if you do not reconcile.