First of all, great opportunity to have such as long stay for a first trip! You will be able to see so much and have breaks! Our fourth trip was like that and it was so nice. I normally would say HS, but with things closing there for construction like L,M,A, and AK now being open at night for activities, I'd say AK. You can also see how you feel after doing the first day if you will need another, or if you have a hopper pass, you can hop back over to do a few things on another day. At HS, I'd recommend getting there for Rope Drop or going to an early EMH day if you are on property, to go right to TSMM before or after you sign up for Jedi Training. It is a lot of fun, and you will want to do it again, so you can get a FP for later as well, since lines are long there all day. I don't know with the age of your kids about RnRC or ToT, but you can go there next when lines are still shorter earlier, or FP ToT for later( RnRC has single rider also if one of you wants to jump on). You can FP Star Tours, and will probably want to go to Launch Bay if your kids are into SW. You can get in line to meet Chewy and Vadar or Kylo Ren, and see other movie memorabilia. They can also build their own lightsaber in the gift shop on the way out of Star Tours.
Indiana Jones has shows throughout the day, as does Beauty and the Beast and the Little Mermaid, and the Muppets. Once you know your Jedi time, you can figure which ones work at which time for your day. Fantasmic is sometimes shown twice a night, so you you can check that as you get closer(may be able to get FP for it as your fourth after you use your 3), and just an FYI that it can be a little scary for younger ones. There will probably be the new Star Wars Fireworks at the end of the night to see as well.
AK has a lot to explore, and especially if it's hot, you may want to not do it all in one day. Definitely want to do Safari first thing, and it's nice to do more than once as each ride is different depending on where the animals roam. Lion king show is excellent with Broadway quality performers and acrobats, and shown several times a day, as well as Finding Nemo show and Flights of Wonder. You have Everest(if your kids are game , or you can do single rider or FP) and Kali River Rapids in Asia, and then Dinoland with carnival style rides and Dinosaur(FYI - loud, dark, and jerky - can be scary for kids). There is also a dig- site playground there.
Then there are the many animal exhibits which you can walk through at your own pace. The two main ones are the Pangani Trail (gorillas and underwater hippo) and Maharajah Jungle Trek(tigers, bats). We also love the Discovery Island Trails that take you up close to the Tree of Life - it is really amazing to see all the animal carvings up close! They are not well marked so you may have to ask a CM. if you have extra time, you can take the train out to Rafiki's Planet Watch, which has exhibits and you can see a vet working on animals in the a.m. It takes a lot of time out of your day to travel on the train back and forth, so wouldn't do it if you only had one day. There is also a fun free activity for the kids called Wilderness Explorers, where they get a book and go to different stations throughout the park to learn things and get badges. If you have two days you can hit more stations and they can get more badges done.
And now they are adding the new night activities including the Rivers of Light show, a parade, and doing the Safari at night with a sunset lighting. So while it usually closed around 5-6, and you could hop to another park, or hit Disney Springs, you may want to stay to check that out. Sorry for the long post, just like to give first timers as much info as possible, since I got a lot of help here when I was a beginner!
cassie58