Definitely MK. Tons more rides fit for the whole family there, young kids especially, and many don't require fastpass. AK only needs 1 day which you already have. You can go to the local zoo (or farm) to look at animals any time for a lot less money. You can't get rides like MK anywhere else. DHS isn't interesting to kids at all. My son, years ago when he was 5, used to call it "The Horrible Studios." He said, "I don't want to be in the Horrible Studios! I want to go on a ride!"
Weird. All kids are different. Sorry yours didn't like DHS. My 5 year old really liked DHS in 2019. Frozen, Indiana Jones, Lightning McQueen, etc. Not sure if any of those are running currently. She loved everything Toy Story Land and especially Star Tours. Both SW:GE rides are also likely ridable by a 5 year old, unless there is a fear factor. Height restrictions are pretty short. Plus the new MMRR ride, of course.
She also loved Epcot, especially Soarin, Test Track, and Frozen Ever After. She enjoyed the various pavilions and performers (which I understand are not happening now with covid). And getting food/treats at various kiosks. Also riding the monorail to and from the park (again not sure if that's running when OP goes).
Animal Kingdom, to me, is unlike any zoo I've ever been to. Although my 5 year old couldn't go on Everest or FOP, we rode everything else. She loved Kali and would've ridden on it many times if allowed. We love the Safari and the various trails. Lots of room for kiddos to skip and hop and discover things. She did some of the Wilderness Explorer challenge. We spent a ton of time at the Boneyard area. (Which was also great for resting my feet.) Seeing the vets work on animals at Conservation Station was one of the highlights of our trip. Again, I know FOTLK and Nemo aren't running, which is unfortunate because they're both great. When it's open, it's one of the best places to meet Mickey and Minnie with short waits. If your kids are highly ride-oriented, then AK might not be the best choice for a second day, but my family could certainly spend more than a day there.
I agree that a young family could easily spend 3 days at MK. Personally I prefer park hoppers and splitting days, but I also prefer mid-day breaks. Other people tour differently, of course. If you do have parkhoppers, people have reported good success getting to DHS in the afternoons. And if you're not taking mid-day breaks, it's an easy hop from Epcot to DHS.