RE: places to take out-of-town kids
The Berkeley Marina might work well for you here. You can fly kites in Cesar Chavez park, and get great views of the bay, golden gate bridge, etc. from there. This is on the right side of the Marina as you are going in; on the left is Adventure Playground (alright for the six year old, but probably too advanced for the 3.5 year old), but there is also some standard play equipment.
Adventure PlaygroundThis is an unusual playground run by the city of Berkeley and if you haven't taken your kids there yet, you are in for a treat. This place is one of the reasons why it's so cool for kids to grow up in the Bay Area! Kids love it and will entertain themselves for hours. In fact, children over 7 can be dropped off for up to three hours - parents fill out a registration form with a phone number where they can be reached. There are trained supervisors to oversee kids as they play.
The philosophy at Adventure Playground is to provide elements that are not normally found in a play environment. There are piles of boards, old carpeting, plywood to make signs and walls and roofs. Kids can check out hammer and nails, a saw, buckets of paint and brushes. There is an ever changing collection of kid-built shacks and club houses. There are old tires lying around everywhere, a huge cargo net to climb up and (my kids' favorite) a cable and pulley slide: you stand on a a wood platform, grab a rope attached to the pulley on the cable overhead, wrap your legs around the rope (which has a tiny seat attached) and zoom down into a pile of sand. Then you walk the rope uphill to the next kid waiting on the platform and get back in line! (Parents beware - your kid has to meet a height requirement to do this.)
Adventure Playground is next to the Bay, at the Berkeley Marina (take University Ave west all the way to the end; go left at the fork and it will be on your left.) When you go, you might want to take along a picnic and take a walk along the Bay - the area where the playground is has a tiny beach, paths, and picnic tables. Nearby attractions are the the kite park, the pier, and the marina.
Hours: 11-5 Sat & Sunday only, plus all Berkeley holidays (daily during the summer). It's free