If you won't have any little kids then advertise that. Have pictures of age appropriate areas in your daycare space for school agers. List activities that a day home that has toddlers can't easily do (a focus on sports and games etc). Will you supervise a homework period to have that done before parents pick up? Will you have outside play?

Somehow word it so it sounds like being closed for summers is a perk. The kids can go to daycamp etc without parents having to pay to hold their space.