This will be really enjoyable trip! I have couple of suggestions. For Plitvice and Krka make sure you are at the entrance at the opening time: it gets really crowded later in a day and it can take away some of the experience. So, for Krka, I would suggest spending night in Skradin, which is next to the entrance. Daytrip from Split is backtracking, as someone said, and you will have to get up early to be there at the opening time.
Also, for the night in Plitvice, you should know that hotels are pretty much in the middle of nowhere, it is like spending night at the lost lake on Mt Hood. If you require some city amenities, I suggest spending night in Bihac, Bosnia. It is larger town, 30 mins drive from Plitvice. Prices there are substantially lower if that is of concern.
For Krka, often you will see these photos of ppl swiming in the waterfall. In reality, it is so crowded, that you can barely swim without hitting into someone. Also, if you want to maximize variety of experiences, seeing both Plitvice and Krka might be redundant.
Dubrovnik and Korcula are good picks, however, as someone mentioned, Hvar is not really known for family oriented tourism.
For an extra day I would suggest something other than Kotor in Montenegro. Thing is, Dubrovnik, Korcula, Hvar and Kotor are all cities that belong to italian cultural sphere, perhaps you want to do something else. If Montenegro is where you want to go, maybe Mt Durmitor and Zabljak town. It is marvelous mountain you can hike, or, since you are with the kid, you can drive through. Lots of opportunities for photos.
Other option is to spend a night in Mostar, Bosnia, which is more along your route (2 hours detour from split - Dubrovnik road. Mostar has ottoman oriental feel which is interesting change.
Are you staying in the old town in Dubrovnik? If yes, your car will become a liability, parking is hellish. I would use big parking garage above old town, but they do charge a lot. Also, going by car to Dubrovnik, you will have to go through Bosnia.
Lastly, coming back to Krka. I love the place, but unfortunately it is way too crowded and this does diminish experience. However, these types of rivers and waterfalls are common in Yugoslavia, so risking wrath of other cro fans, I would suggest that you simply search for some other river in the region, less visited. What comes to mind is Kravice waterfalls in Bosnia, also karst region river where you can swim. It also has bunch of tourists but still, they are at least half the number of Krka visitors. And they arrive after noon.
Hope this helps.