Six year old?
https://beachboardwalk.com/
If your six year old likes nature, could get a tent-cabin at this place:
http://www.costanoa.com/
if you want some mild adventure. Lots of nice walks around the resort property.
Visit Pigeon Point Lighthouse, very close to this resort,
https://en.m.wikipedia.org/wiki/Pigeon_Point_Lighthouse
And a little hike out to see the Elephant Seals at Aņo Nuevo State Park, again, close to this resort and Santa Cruz.
http://www.parks.ca.gov/?page_id=523
You'll have a great time. Perfect time of the year to visit.