Oh my, I am so glad to have found such an amazing place in the area. It is well hidden - at least from all the people that I know :)
My son did not stop running around and trying every single thing: he was on the train ride 3 times, climbed in the old town several time, went into the whales mouth I don't know how many times :) it was great to watch him enjoy like that.
The park is probably appropriate for kids up to 7 and my son is 2 and a half. It is most definitely worth the trip to Oakland and the park is scenic so you can go on a picnic or just enjoy the view.

It is definitely one of the best parks in the bay area.
Parking could be a bit tricky - if you don't see the steam train then you have probably parked around the corner which is no big deal but there is an option to park close by so ..
We had fun especially with the Steam engine and the slide. Other than that some of the activities are not for my 2 and a half year old.