My 4-year-old son and I did various parts of Coyote Creek Trail, but this is the best route with good amount of shades from trees. Further north, there is too much trash. Further south, it's really noisy being too close to 101. 

Another pretty easy ride to do with kids. There are a couple of kind of steep hills where kids may need to walk their bikes, but it's mostly flat. The one big downside is that this trails VERY BUSY.