DC Bart to GGP is probably the best bet. From DC Bart drop down to Westlake, around Lake Merced, great Highway and viola @ GGP. Great Highway has a nice bike scenic bike path once you pass the Zoo. Riding back requires a little uphill.
http://www.gmap-pedometer.com/?r=2898126
John Daly Blvd aint great, but its plenty wide and once your over the freeway you can go a street over. Also, sinc eyou have kids the sidewalk down is an option.