I don't live in LA, I'm in Orange County and I don't go up to LA if I can help it, but...
In my experience Santa Monica is nothing special. A rather dirty beach town with the usual homeless folks and street people wandering between the generic mall shops and tourist traps of Santa Monica Promenade. There's a touristy/urban mall strip like this in most big cities.
Santa Monica Third Street Promenade
Venice is a broad expanse of sandy beach with the usual dinginess, graffiti and gang activity acting as a backdrop to some admittedly unusual street theater. Nothing you couldn't find at any big city urban beach, except for generally better weather than the East Coast or Great Lakes cities.
Venice Beach "Boardwalk" (but made of cement, instead of boards)
Instead of doing the beach thing, and if you absolutely need to stay up in LA instead of heading down to the safer and more pleasant OC beaches, what about heading 10 minutes further up the 405 Freeway to The Getty?
The Getty Center - Los Angeles
There's likely an exhibit or two there out of the dozen or more that should interest at least part of the family. Or just riding the monorail up the hill and having a meal on the terrace with the great view and strolling the grounds and gardens and pavilions and gift shop is a fun activity even if there aren't any art or architecture lovers in the group. Plus, aside from parking, it's entirely free and all compliments of The Getty Oil Foundation. Thanks J. Paul!
http://www.getty.edu/visit/hours/map.html
