@Socal Labrat just looking at the temperature on Tuesday, August 3rd. It will be 89 degrees as a high but only 70-81 degrees during the sweet spot of the park for rides from park opening 8am to 11am. I'd plan to get alot during this time while the kids still have a lot of energy and the summer heat hasn't really kicked in yet.
First, looking at the GPS, it's 61 miles from Carlsbad to Disneyland with no traffic. I'd honestly leave an hour earlier to factor in traffic and walking from the parking lot to the parks. So if you want to be there from rope drop at 8am (which you do with only 1 day at the parks) I'd leave Carlsbad the VERY latest at 6am. Give yourself that 2 hour cushion for the drive, traffic, parking and walk to the parks. Although honestly, if the kids are up for it and can sleep on the car ride over, I'd leave a bit earlier than 6am to be safe.
Parking wise, there are no trams from Mickey and Friends parking structure, so if you park there, you'll have to walk the full length of Downtown Disney. Personally, I'd park at GardenWalk and use that short walk to the Harbor side entrance. Good thing about GardenWalk too, is it's open 24 hours, so you can get there as early as you'd like.
If you get there at 8am, tackle Peter Pan and Snow White first. After those two rides, most all the other Fantasy Land rides should be 5-10 minutes each the first hour or so the park is open. Goal is to knock out all the Fantasy Land rides in the first hour to 90 minutes of park opening.
After that, I'd head to Toontown, and spend some time there. Take pictures with Mickey and Minnie as they usually are near their houses (you can't enter the houses but they're there for character pictures). Plus do the two Toontown rides with moderate low ride times around 9:30 to 10am.
After that, I'd take the Disneyland Train from Toontown (cool experience for their first visit) to New Orleans Square to do Disneyland classics like Pirates of the Caribbean, Haunted Mansion, Splash Mountain, Big Thunder Mountain, Jungle Cruise and if your grandkids are up to it, Indiana Jones.
If you factor in a visit to the Enchanted Tiki Room, Dole Whip and maybe lunch at a quick service place like Hungry Bear or Rancho. This should take you up to 1pm-2pm timeframe and time to park hop to DCA.
Problem with DCA, at that time, the sun is full blaze and not enough shading and all the big attractions will have 30-60+ minute wait times with Soaring, GRR, RSR, GotG. No good way to handle that but than to pull the bandaid off sort of speak, and tackle those long lines. I'd recommend umbrella's and lots of water for the kids while waiting in lines.
If you're grandkids are big Disney fans as you said, they'll enjoy Mickey's PhilharMagic. They recently added a new scene from Coco and it's a great break to take on a hot day with comfy seats and air conditioning.
Once you tackled those long lines for those rides, that should take you close to 7pm or so. I'd hop back to Disneyland at this time but I'd stop by Smokejumpers and get a quick bite to eat with either Monte Cristo or Brisket Mac and Cheese offering they have. They also have a decent hamburger.
Once back at Disneyland, you have about an hour or so of rides to do before the fireworks begin at 9pm. So I'd recommend Tomorrow Land and your grandkids would like Buzz Lightyear, Autopia or Star Tours if the lines aren't too long. Check the app to see which one has short lines.
After that, goal is to get near Small World, where you'll get a great view of Mickey's MM projections on the face of the Small World ride and be close enough for the fireworks, which are launched from Toontown close by.
After the fireworks, around 9:30ish, if your grandkids are up for it and not tired. It's a great time to walk through Star Wars: Galaxy's Edge at night, it's really stunning with the night time sky and lighting. Perhaps even do MFSR ride before park closes.
After that, you'll be thankful to be parked at GardenWalk, instead of trying to navigate a very packed and crowded Downtown Disney to walk back to Mickey and Friends parking structure.
Atleast that would be my plan if I were in your shoes and hope some of these suggestions help give you and your family some ideas.
Hope your grandkids have a magical first visit to Disneyland!