For one start in each park @Julianne_fki’s starting suggestions are exactly what I’d do.
Guardians is a great RD option if it’s a ride you really are excited about it, because you’ll likely want multiple rides on it (for fun and potential variety in different song) and can only get 1 LL.
The walkway is closed right now between Avengers Campus and the back side of Carsland, so yes exit Avengers Campus (I recommend going past Webslingers so you can see the main entrance, I especially love the music there) and then turn left on the main walkway and you’ll get to the Carsland entrance.
Being eligible for Early Entry, there may be specific roped pathways to use to keep you ahead of the rope drop crowd that will be being held at some point between the Carthay Circle hub and entrance to Avengers Campus, so pay attention to the various CMs and signage as you go.
But a definite perk to early entry is that you can be positioned to knock off 1 blockbuster/E ticket ride with no wait and then if that works, still have time to queue up to have the same effect for Racers (assuming it opens on time with the regular park opening). The regular rope drop crowd has to make it all the way from their rope to the back of Carsland before park open so you have a definite advantage being able to position at the ride entrance before that.
And I would recommend using the last 10 or so minutes of early entry to get into position. If it doesn’t open within 15-20 minutes of regular opening, pivot and use the LLSP to get one guaranteed ride on it. It’s a fun one that is easily a top choice for most people to re-ride, but it’s definitely tricky to reduce waits for re-rides since. One option is single rider (but your 4.5 yr old wouldn’t be able to do it but maybe one adult can take them to one of the other Carsland rides that are a hit with that age group while the rest single rider for a repeat ride on Racers).
And then for a DL start, definitely Fantasyland for one day only. We love to start with Alice or the Boats first. My older kids will usually then break off to Matterhorn while we take the little kids on Dumbo. Then we’ll get to the boats if we haven’t done them yet and finish up with any of the other 3 dark rides (Snow White, Pinoccho or Toads), that we feel like (again skipping PPF), and the carousel and Casey Jr. Train. If you skip some of the above you’ll still be able to hit MMRR in Toontown before waits build (or hit the ChipNDale Gadget/GoCoaster first if it’s still a 25 min wait or less if you have an LL for MMRR). I would also check out the lines for Mickey’s and Minnie’s houses before they build.
And then I usually try to have an “exit” plan for Toontoon since my kids coule play there forever. So either a show, or meal or LL for small world lined up to keep us moving to the next step and promise the kids we can come back in the evening if there’s time when lines are atrocious everywhere and play more then.