If you live in or near Berkeley this is a waste of a day trip. It certainly is not worth trekking to. There is very few things to do and not much room for children to roam. There isn't much to see except scenic views. Plus there are some creepy people who hang out at the pier.
Extending 3,000 feet out into the bay, the pier is a good place to stroll, take photos ofthe skyline, and watch freighter traffic. Many people fish for perch and flounder, and you may see sharks and stingrays caught too. The dependably breezy waterfrontpark is the site of the annual Berkeley Kite Festival in July, when amateur and profesional kite flyers compete. There is a 1.7-mile loop of paved walking trail, a plaground, and room to roam.
There is a great walking/jogging trail that takes you through some spectacular views of the Golden Gate bridge on a clear day. The wildlife is spectacular and abundant. There's a cool kite festival yearly, too. I watched kite surfing while there a few years back...definitely cool.