Take the metro. Get there early. And have oodles of patience. Bring sunscreen.
I guess it depends on what sort of experience you want and where you want to be to see the fireworks. If you or your family kayak at all and aren't interested in the sitting on the mall listening to bands experience - consider renting boats and just watching the fireworks from the river. Just keep in mind to raft up as a group and realize that there will be a lot of drunken power boaters on the river with the same idea (watching the fireworks)...