My oldest four were born within six years of each other, and we traveled extensively. The first three were on airplanes before turning a month old, which seemed normal at the time but crazy when I reflect back. I had my oldest and youngest buddy up, and my middle two buddy up. Your older two are perfect ages for it. Basically they keep an eye on each other while you're distracted with the airport and logistics and stuff.
1. Dress the kids in the same color - shirt, pants, either, both. It just makes an easier visual in the chaos of the airport. Take a quick photo before you leave the house/hotel, or at the airport when you're checking in. Heaven forbid a kid wander off there or while on an outing, it's just nice to have a very recent picture in your phone.
2. Pack a grocery bag or kitchen sized trash bag (non-perfumed, preferably) in your purse/diaper bag. You'll use it for any diapers you change at your seat, food trash (banana peels, granola bar wrappers), baby wipes used to clean (definitely wipe down the tray table), tissues, and potential "Sweet Mother, here's the inflight magazine just rip it to shreds" moment when the kids are bored or their ears hurt and they're borderline insane. This lets you leave your row relatively neat and keeps you from bugging the flight attendant. Just hand her your one bag at the end of the flight.
3. When the flight attendant comes through with the cart, request 1-2 empty cups. Babies love these. Big kids can use them to hold snacks. Bigger kids can use them to hold crayons. Don't plan on drinking anything hot. In fact, with a baby in your lap I wouldn't plan on drinking anything she serves. You're better off bringing your own bottle (water, soda) that you can keep a lid on and stashed in your lap or the seatback pocket. Drink your coffee/tea in the terminal or at home.
4. Fussy babies love the air vents. Just stick their face in front of the air vent for a second. They're forced to inhale, and thus to stop fussing/crying. Make a game of it. Hopefully you brought a sweater for yourself. LOL
5. I bring a travel pillow - especially nice if you'll have baby in your lap the whole flight. Armrests start to hurt after awhile, and babies never seem to be comfortable in the positions that are comfy for us, am I right?? Before we board I shove a paperback or magazine into it, in vain hopes I'll be able to read something. (Note: I pack a second pillow case to use at the hotel. Airplanes are dirty. For our return trip I use the first pillow case, inside out.)
6. Older kids each bring a backpack they can carry. We put in a few books, snacks, non or low-noise toys, headphones, and blanket. Even if they don't get cold, I've used those to clean up vomit, as a barrier for potty accidents in the seats, as sun shades, to block cold at the window seat, and when my boys were younger they had enough space on the floor to play trains. Those were the days! The most important thing to pack: plain paper tablets. We can draw, write, tic-tac-toe, fold origami, paper airplanes, so many options. Also it's something I can do half-asleep or with a fussy baby on my lap. One of our easiest "games" was to each think of one item and then we each had to draw a picture with it. So I picked a tree, Son 1 picked a dog, Son 2 picked apple pie, Son 3 picked a soccer player, etc. and we each drew a picture that incorporated all four items. Then we shared with each other. That bought me about 10 minutes at a time, sometimes more. At 4-6 I brought crayons, but older than that I found colored pencils easier to carry around and use.
7. Set expectations ahead of time to reduce potential for whining/anxiety. My routine was to look around the airplane during boarding - where is your nearest exit? Where are the bathrooms? What is the F/A doing? What are the ground crew doing? What does the emergency card say? How do your lights work? Killing time stuff. I made them pay attention to safety and crew announcements, just a respect/habit thing. We read books from push back to the first ding. At that point I let them get into their backpacks (nowadays I'd let them on their iPads). We'd have a snack at the level off announcement or beverage service, whichever came first. Anything goes during flight. At the "We've begun our descent" announcement we'd put away everything in the backpacks, including electronics. Then we had our second snack. At the "prepare for landing" announcement we handed over our single (full!) trash bag and read until landing. Many times I read aloud, sometimes they read on their own. Once we got to the gate, we only had one book to put away. The kids did that while I got baby into the sling.
8. Some people like a stroller. I didn't. But if you want one, stick with an umbrella stroller and see if the crew will let you stow it in the front closet.
9. Some people like the bulkhead seats. I didn't. I really needed the storage space from a row in front of me. PLUS these days there isn't much room for kids to play anyway. Your kids are old enough to not kick the seats in front of them, and the baby young enough that he won't intentionally do it.
10. Make a note of CA's laws re: car seats. I believe it's 8 and under need seats of some kind. Renting a car is your best bet. Buses are another option, I'm not sure if SD's buses hit the tourist areas but it's worth looking into or asking here.
Your kids are a perfect age to do this! It can feel overwhelming, but I promise it's not that big of a deal. Just have a plan, make sure part of that plan is to be flexible, and enjoy your trip.