This is a tough one for me. We have done WDW 3 times now and just did DL for the very first time in June. Normally when we plan a lengthy family WDW Trip, we split MK into 2 days, cutting the park down the middle, doing one half each day. Our recent visit to DL had us fall in love. We felt that for most duplicate rides/experiences, DL had higher quality, smoother, more immersive experience than WDW. Doesn't mean WDW is bad in any way, but that with so much space and options at WDW, there is more of a quantity vs quality dynamic in addition to DL resources on congested SoCal being focused more on upkeep/maintenance than expansion.
So, you MAY find that duplicated rides/attractions may be a bit lesser of an experience than you are used to. That being said. Take full advantage of the quantity factor. There is so much more available to do. While in MK, try to do the things that are different than in DL. That might also mean preparing your feet for larger scale between lands.
We were last at WDW for a 12-day trip over Thanksgiving and our next family trip won't be until 2021. However, in the meantime, we have a quick 2-day visit before a
DCL cruise and my wife is surprising my middle son with a just-the-two-of-them trip for about 4 days in October. With five of us ranging from 7 to 15 currently, we find that longer trips allow us to intersperse break times and go at slower pace and experience different things away from everyone else's bustle. It is significantly more expensive this way, but that's why we can only do that with a few years break in between.
Honestly, the more I think about it, really look at what your kids would love about the other parks to see where you may want to spend more time, especially DHS. The FP+ system can be great for some things and harsh for other things.
Epcot and DHS are tiered while MK and AK are not. This means that the higher profile rides at the tiered parks, especially DHS, are limited in choice. Choosing DHS for 2 days instead of one MAY provide a better opportunity to experience Toy Story Land more completely. However, the same can be said about Epcot, too. That just depends on your kids interests: Toy Story, Frozen, Star Wars, etc. If princesses are their thing, then I still wouldn't rule out a second MK day, including maybe a meal at Be Our Guest or Cinderella's Royal Table.
Hope this helps give some thoughts or direction.
Just some thoughts comparing the size and available attractions at the parks. Also, the demographics of DL being significantly more locals allows those to come and go as they please, able to come back another day. At WDW, this percentage is significantly lower, with most people doing a trip like yours - once every few years, and trying to get as much in and as much right as they can. So while most Disney fans are nice and respectful, you may want to mentally prepare yourselves for some who feel entitled to get what they want because of how much time and money they put into planning the perfect trip. IOW, people can suck sometimes. So, you CAN set this up to overcome some of this if you spend your MK day leisurely experiencing the differences from DL, but especially the nighttime show. We have always found AK to be the most relaxing for us as a family. This is probably due to the lower number of thrill experiences and more streetmosphere than the other parks.
Take your time, soak it in, do what you can, and think of things to plan for your next visit. On your next visit *wink*, plan more non-park days, where you can also do lots of fun activities at different resorts (some free, some paid) or even just explore the various resort theming, amenities, restaurants, etc. Those are great ways to break up the rush-rush of busy park days.
Most of all, enjoy!
Ha! Actually, depending on how/where you got your tickets, you may find it cost efficient to add more ticket days. By doing so, you give yourself flexibility to go back to a park just for a fireworks show if your kids got tired early and you had to miss it. Or if you are visiting/exploring a resort near a park, you can duck in for a meal or just see something you may have missed, just for an hour or two. The price per ticket day goes down the more ticket days you have. When we get to 7-10 days or so, it's only 20-40 bucks sometimes to add a park day. Look into that. It provides flexibility and the opportunity to increase your FP+ chances, especially for tiered parks like Epcot and DHS. You don't have to spend all day there. Half-days more times can be helpfully relaxing too.
