The MK closing time may depend on whether Disney continues with Very Merriest After Hours or switches back to Mickey’s Very Merry Christmas Party. In 2019, I think MK closed at 6:00 pm on the Christmas party days. Since WDW is bringing back Mickey’s Not So Scary Halloween Party wiith 6:00 pm MK closings this fall, I think there is a decent chance that the Christmas party will be back too.
We shall see. Just something for the OP to keep in mind.
This is all incredibly helpful, thank you all.
In terms of timing, what are the critical dates to know? For example, when are the hours the park will be opened released? That seems like a crucial influence for what days you would want to go, and therefore, what reservations you would want and which days. I know you can book restaurants 60 days beforehand, but what other dates should I be looking out for?
I think it has been at least 75 days in advance recently. Pre-Covid it was more like 180 days in advance.
But if you have already purchased your park tickets, I would suggest making a tentative schedule and booking park reservations for those days. Currently you can switch a park res to another park res that has availability (on a date your tix are good for). I already have park res made for my fall WDW trip.
We had our 3 great granddaughters on a trip in September 2019, ages 9, 8 and 5. None had been on any amusement park ride. I think maybe the train at the zoo.
We had discussed testing ahead of time - maybe you tube videos. In the end we just winged it.
As it happened our first WDW morning was MK so first ride was Aladdin’s Magic Carpets. The girls screamed in terror.
Then we did Pirates and Splash - never mentioning anything, just going on them. The girls loved them. They eventually rode Tower of Terror just fine. The 9 yo tolerated RnRC and EE but loved Dino. The 8 yo had her arms in the air on RnRC and EE and was terrified on Dino. None would agree to repeat Magic Carpets. 
But again, we were several adults and few kids. We went and they followed along.
The hours are posted far in advance, but they frequently change as the dates draw closer (in my experience, the changes generally add hours, unless they are adjusting for an after-hours event). If you’re tracking your trip days on Touring Plans, you’ll get emails any time the hours change - I love those emails because I usually wind up squealing with delight seeing that I’ll get extra park time because of the changes!
I’d see which park is open latest, probably MK, and do that on your first PM half day. I would also put my rest day as in the middle as possible: so three park days, rest day, three park days. I’d also use the crowd calendar to choose which days I went to which park. And with littles as young as yours I’d focus on MK first and AK second.
What do you think about attending the “after the fireworks” MK dessert party? My biggest concern would be staying out too late for the littles (right now, they are generally ‘good’ until about 9:15, and then they start to meltdown).
The reasons the after the fireworks appeals is because (1) from a seating and cost standpoint, that seems like the way to go, since you would get priority seating outside and (2) I hope we would be at the tail-end of the crowd departing, so it would be easier to get transportation back to the hotel (though I may be delusional and the crowds could be just as bad).
So the main questions would be
TIA, as always, for your A+ advice!
There is no seating in the garden viewing area, only artificial grass to stand, sit, or lie upon.
DH and I did the after fireworks dessert party, mainly for a good viewing spot that was not packed like sardines. We checked in early and claimed a spot by the back fence, then sat on the ground to rest. Kids can run about in that fenced-in area. When things started, then we stood up to watch and didn’t have people right in front of us. After the pyro, we could get our treats and sit at the tables to rest and enjoy them at our leisure.