You can always reach out to Costco although their terms and conditions say to contact the pick up site for full additional driver conditions but it's been my impression that the basic rules of a rental car company are not removed when you book with Costco, Costco just has a deal arranged with these several rental car companies.
The Costco rental includes "One additional driver fee is waived for Costco members at participating locations in the U.S., Canada, UK, Puerto Rico, France, Germany, Ireland and Spain." Alamo does not charge an additional driver fee for a spouse or domestic partner. It would not make sense to waive a fee that isn't charged to begin with so my impression would be OP would need to declare their daughter as the additional driver and their spouse would still be able to drive for free. Again call the pick up site if you want to speak to someone first before making the reservation as I would not take my word as what it is.
All the car companies Costco has a deal with (Alamo, Avis, Budget and Enterprise) include the spouse or domestic partner without an additional fee.