Mike's idea about Flightline is a good one, and here's a suggestion for a loop in and around Los Pen you might really enjoy with your kids...and I'll set it up with the most singletrack.
Trailhead at the west, Sorrento Valley side, using the maps on the Gary Fox website I posted above. Ride east on the Main Trail, south of the creek, which is a fire road. After the last of your four mild climbs, you'll come to the sign marking the Waterfalls, which is the three mile mark. You can stow your bikes in the wood racks there, and walk 75feet down the trail there, to the water and the cascades, which run all year. Good place for a snack.
When you get back on, continue east on the fire road (skip the singletracks marked closed to bikes) for another mile until you come to a fork in the trail with the main trail being the right fork, and Carson Crossing being the left...which you'll take. You can easily ride the second of the two water crossings here (all of 6" deep) and should go north, out from under the shade trees, taking a right when you hit the T right after the water, and then head North on a fire road that leads uphill past a 4-way intersection. That fireroad climb will take you to the top of the mesa, and when you come to a T, go left/westerly on it. When you get up on top of the mesa, continue west on the fireroad until it comes out onto pavement. Then, continue west on the pavement to the Tee, hang a left and go straight south on the pavement onto the fire roads on top of the mesa.
Once you cross over the gravel and onto the dirt, you'll pass onto another fire road that I suggest you take westerly across the mesa. On your right, you'll see singletracks that you're going to come back on. Take the fire road until you come to see some cut, but undeveloped house pads, at the bottom of the very last Vee just before those cut lots, hang a northerly, downhill right down the fireroad that exits the bottom of the Vee. About 200 feet down the fireroad, where it bends, there's a singletrack that leaves to the right/east which you'll take up a gentle climb to a Tee intersection on singletrack--turn left.
Now, you're on the network of singletracks that you bypassed as you rode west on the fire road, and you can just follow and take every looping one of them. The go out to the points on the mesas, and there are a few spots where you can show off to your kids as your ride down drops, and then they continue looping back to the east. The second-to-last-one pops you back onto the fire road, and the last is just up on your left as your ride north on the road. After you ride the last one, which goes right along the fenceline of a nice home at one point, go straight across the fireroad when you exit the trail. Take that road width trail due east to the trees, then bear left on it to go north. 300' up, just before the road turns west at the fenceline to another development going in, the Cobblestone trail (unmarked) leaves down to the right. Take that singletrack. Just around the second corner, you'll see a Kiosk, and that singletrack descent will be really fun. Keep an eye on the signs --there are some side trails closed to bikes, but the main singletrack is open to you all the way back to the Penasquitos Canyon floor--and you'll get some whoop-de-do's on the way.
When you come out at the very bottom, take the singletrack southerly toward the pointed rock pile --that's the Waterfalls area you came in at, and you've got about 11 miles so far. At the north side of the waterfalls, hang a westerly right turn, go about 150' and take a left on the singletrack that forks off the fireroad ascent. Now, you're on Bridges Trail, and it's a fun roller coaster that will take you along the north side of the creek to recconect with the fire road going west. The fire road will devolve to double track and will roll up and down a little before you pass a sign for "Wagon Wheel Crossing". If you're tired, take Wagon Wheel to the south side, hang a right and you're just about where you came in.
If you continue straight past wagon wheel , the trail goes back to singletrack...you can take that westerly all the way to a fire road then another half mile to where it pops up onto a street: turn left and ride the sidewalk south over the bridge, then another hard left onto a paved singletrack that goes back easterly along the back of a business park. Just keep going easterly until you have no choice but to hit the sidewalk...and you're in the Cantina Lot marked on the Gary Fox maps. If you parked here, you're done, if not---ride easterly on the road up around the corner to the big parking lot.
15 miles, probably, and I left out the sketchier descents and harder climbs, so if you liek what you find, you can mix-n-match: it's very easy to reconnoiter and figure where you are around there.