diphthong

yeah...done it multiple ways from la to san diego but nearly always through camp pendleton which can get a little busy at the south end/entrance with freeway on/off ramps.

if it's gotta be in may, that kills (as said in the op's og post) off palm springs and phoenix (and death valley and the tucson loop) which could all be very doable in early/mid april.

it could be done from (as mentioned preciously) from just south of malibu-santa monica, starting along the bike path-to the "enforced" edge of camp pendleton at las flores rd.
only one adult chaperoning rider is mentioned...i'd want two-one ahead and one behind for certain stretches such as the exit from the bike path in redondo and transitioning
into palos verdes estates (your longest grade), along with the rancho palos verdes into san pedro, dropping down into the la harbor area, negotiating anaheim st until the la river bike path into long beach
segment as well as the mariners mile along pch in newport beach. the path would take you inland along the newport back bay trail (climbing out onto jamboree rd sidewalk would be
your steepest short stretch the entire ride) and take advantage of the irvine bike path network to deposit you as close as possible to the corner of trabuco rd/marguerite pkwy.
from there, it would be staying along the south side of the 5 fwy in that corridor until ortega hwy/hwy 74 where you would make a quick left just before crossing over the 5 fwy,
then a quick right to them hook up with the san juan capistrano bike path that deposits you at doheny state beach. from doheny heading south thru san clemente and utilizing the well-signed
and rolling bike route south of el camino real and the 5 fwy until about a mile north of the trestles bike path access. easy ride past trestles and the san onofre nuclear power plant
and enter san onofre state beach area. once thru that and onto the northern part of the (usually) open camp pendleton, it's another few miles to las flores road junction where there is a
large turnout and plenty of parking just east of the 5 freeway/las flores intersection. a couple of thoughts on this section besides wanting a lead and trailing adult rider in certain areas
if doing this ride:
1. no matter how you zig and zag, it's a little steep (and long) for an 8 yo to climb from redondo beach up to palos verdes estates, then up (easiest) via del monte to via montemar and then a right
down to the bluff cove area within the space of 3-5 miles.
2. taking into consideration the various bike paths you pass north to south (ballona creek, la river, san gabriel river trail, santa ana river trail), you would only be on the la river trail for approx
2-3 miles. there are some situations along there that i wouldn't feel comfortable exposing my kid(s)-mine are grown now-to at that age. you could incorporate some part(s) of
the sgrt and or sart as a there and back or starting/ending point but those two trails, along with the la river trail i would save for 16+ yo's. haven't had any such vibes on ballona creek,
the irvine trail network (including back bay) or the sjc to doheny path.
3. if i mock up and dump the ride north of long beach and go all the way to las flores, i come up with 85 miles by doing much of the balboa peninsula and jutting inland with the back bay
trail and following the relatively gentle trabuco rd/marguerite pkwy corridor to sjc (which is a little quieter and wider than the other side of the freeway). noodling and doodling within the
irvine trail network might net you another 6-7 miles w/o repetition and/or doubling back.

i mocked up a san diego coastal century ride that isn't as dicey but could still use a second chaperone here and there. the good news is that it's got about half of the elevation gain.
the bad news is that you repeat about 70-75% of the ride, albeit in the other direction, and it's a more complicated route to follow. the strava route says less than 1,100 feet in elevation
gain but my experience tells me it's closer to 1,500-1,600 ft. still less than half of the la to oc ride where much of your el gain is in the south redondo/palos verdes peninsula area along with the
trabuco/marguerite stretch. the coronado stretch along the silver strand state beach nearly always has a 3/4 headwind heading north for approx 6 miles and the fiesta island loop heading
west and south usually has a decent headwind for approx 1.5 miles.

https://www.strava.com/routes/2799935935163360562

a question is what roads/routes has he ridden (conceivably, orange county) in preparation/doing 50-mile routes so i/we have a better idea of difficulty/traffic exposure?