The California Science Center is open daily from 10 am - 5 pm and only closed for three major holidays (thanksgiving, Christmas, and New Years Day). The center is also FREE to all guests, but donations are appreciated. Parking, however, is not free, but will only cost 8 dollars for a full day of science adventures.
In my opinion, the California Science Center is for elementary school aged children. They do have two "discovery rooms" for toddlers if parents chose to take their younger children as well.