I’d leave it unplanned. We usually try to save the last day of a trip open. This way whichever park we end up needing extra time at, we can revisit.
Every trip it seems like we end up wanting to revisit a different park. Sometimes a park is extra crowded, an adr goes long and we miss a must do show or parade, someone gets sick, or we get rained out. Sometimes a park has a lot of new attractions we haven’t tried yet, or we discover something new we really want to try but can’t fit in our original plan for the park. Having an extra unplanned day allows us to make up whatever we may have missed.
If you are set on picking a park in advance, I’d choose based on your interests. We’ve done more than one day at each park, and we’ve done less than a full day at each park as well. It really depends on where the interests lie of the people going.
Once SW and TS lands open at HS, that park could definitely be a two day park because of the large crowds it is going to attract.
AK is fun for a second day to see all the animals and do the Wilderness Explorers badges which your 7 year old might be interested in. Doing those two things alone would take up a whole day.
Epcot is large enough that you can do 2 days there as well.
Another option is going to a water park or having a rest day at the hotel. Sometimes a break in the middle of the trip to recharge can be the best use of an extra day. Plus young kids usually really enjoy spending time at the hotel, swimming, the poolside parties and activities, and other entertainment offered. To this day, if I ask the kids what their favorite part about Disney was, they will tell me the day we got rained in, and they spent the day playing in the Community Room at Kidani.
We normally do 2 days MK, 1 day each at the other parks, and 1 day unplanned. If we have more than 6 days, we add in rest days and water park days.