If you yourself are renting the car then you would have to check to see if your own personal automobile insurance policy would cover you in a rental car in California -- for either liability protection, Loss and/or Damage or hopefully both.
Assuming your two daughters were signing on to YOUR rental contract as authorized additional drivers, they would automatically be covered by whatever insurance you have/purchase (provided all drivers confirm to all terms and conditions of the rental agreement, sign on to the rental contract and meet the minimum age criteria, credit identification requirements and drivers license requirements.)... they do not need to provide nor purchase their own Hertz insurance options. Note: There is no extra fee for additional drivers in California.
If one of your daughters will be the one to rent, then they can check for possible coverage through either their own personal insurance, coverage through a credit card, or optional Hertz car insurance options that may be purchased if needed or wanted.
When renting in California, Hertz provides no liability protection inclusive in the rental rate (as is common in other sates). The renter would either need to use their own personal automobile insurance coverage to cover liability during your rental, or purchase liability coverage from Hertz.
Among the Hertz insurance options that will be available to whomever is actually renting the car are:
Hertz additional liability protection, which becomes primary coverage if purchased. This is only a required purchase for a renter who has no personal automobile insurance coverage of their own.
According to the Hertz Car Insurance information provided on their site:
Liability Insurance Supplement (LIS), if purchased, will be primary coverage for the renter and extra drivers (only drivers who are approved and authorized). It does not cover personal injury or loss of or damage to the rental vehicle.
Hertz will also offer coverage for the rental vehicle itself. This coverage is not mandatory. Loss Damage Waivers are an optional purchase.
Pricing and availability for various Hertz car insurance options will vary by location and are made available at the time the rental car is picked up.
Pages from this site, relating to this topic:
Car Rental Policies
Rental Car Insurance
Personal Auto Insurance Coverage
CDW and LDW Coverages
Car Rental Credit Card Coverages
Personal Protection Offers
Car Rental Liability Insurance