Delta is having prices from Toronto to San Fran and San Diego for as little as $249CDN return or $151CDN one way, taxes included from now 'til August 2017 with LOTS of availability, BUT here's the bad news. For certain dates, you leave at 4:15pm out of Pearson, with a 3hr23min layover in Salt Lake City, and then arrive in San Fran at 10:51pm--which I assume is 10:51EDT, not 10:51PDT. Either way, with a young family, that's a nasty time of arrival, but for that price, it may still be worth it, and if that's not an issue, go for it. Depending on when you're going, you may have decent hour flight times but you always have to stopover--whether in Minneapolis, Detroit, Atlanta, or Salt Lake City.
You can book it on Delta.com or Priceline.com (with Priceline, you can cancel by 11:29pm the following business day if you change your mind). You can check dates easier on Google Flights to San Fran or Google Flights to San Diego, but with San Diego, flights at those prices aren't available after June 2, 2017.