If you are comfortable with the increased liability of a pool used by guests, you can list it as an amenity and add the restrictions to access. Under Booking settings, rules, amenity limitations.
It would be important to reinforce the limitation, especially during a time the guest might see the pool as a very desireable feature, like summer. During the booking process, reiterate the limit and have the guest agree with it. Otherwise a disappointed guest is not going to enjoy the stay and the review will reflect it.