Croatia/Slovenia itinerary help

There are many ways to change, I would take a day from Dubrovnik to add to Istria keeping the car for this. In high season it can be difficult to get accommodation for less than 3 days, figure out that first.

Cro, good point about the 3 night minimum. Thanks for the reminder about that. I think you are right about keeping the car for exploring Istria. Thank you!

About your intention to arrive at Plitvice...

Day 7, see more of Trogir/Split....no rush to Plitvice, just want to sleep there for early arrival to the park
Day 8 early arrival to the park, sleep in Plitvice

I think it's a mistake to time your arrival at Plitvice in the morning. That's when everyone shows up. Like other places across Europe that are tourist day-trip magnets, being there between 10 am and 4 pm means you spend your time there when it's most crowded - the opposite of what you want. The best way to beat crowds is to arrive when they are all getting ready to pile into their cars and buses to leave (say, 3-4 pm), then you get to enjoy the place relatively crowd-free until closing time/sleeping time. You can cover about half the park this way. Sleep right there in the park in the park hotel. Get up early the next morning and charge out, enjoy the park crowd-free (do the other half), then by late morning, when all the day-tripper buses roll up, that's time for you to move on.

One full day, spread across two half-days, is enough time to get your fill of Plitvice and see it at its best.

This same strategy works for most places that are more crowded than you would like. It ensures you get the best possible experience.

I definitely agree with David about Plitvice lakes. However, you are going to one of the most touristy spots in Croatia and in mid summer so you will most likely encounter crowds no matter when you go.
As far as Dubrovnik, spend evening on your arrival day in the Old Town (you will see, Dubrovnik in the evening is a completely different feeling) and one full day for sightseeing (walls, mt.srdj, museums, restaurants etc). Third day in Dubrovnik use for a day trip to some of the islands. The most convenient would be Lopud island or Korcula.
I think sleeping in Trogir instead of Split is a great choice.

All in all you mentioned your family is used to the demands of the travel and I believe this itinerary is almost perfect.

It sounds like a good quick survey trip. We agree with Kate about the evening magic in Dubrovnik. The locals parade down the Stradun and dine leisurely at their favorite restaurants. Join them. David is right - late afternoon & morning for Plitvice. Use the extra day for Rovinj. If you don't find enough to do in Rovinj itself, day trip to Pula or Motovun. In Kobarid, make time to visit the small but excellent WW1 museum. Most of us never learned about the extensive & deadly fighting in the area. Ljubjana is a charming town with good food & markets. A good place to relax a bit before a long flight.

I agree with the idea of mid-afternoon arrival at Plitvice so you can see about half the park late on your arrival day. I believe they sell 2-day tickets. If you are staying at the hotel in the park, they may be able to endorse your one-day ticket so you can return for a second day, but I'm not sure about that. I believe you can take the longest of the marked walks in considerably less than eight hours. I took the second longest during day-tripping hours ago (a nightmare even in 2015) and finished it in much less than the posted time. There are some additional paths (no doubt much less crowded) that aren't part of one of the labeled routes, so you can certainly spend extra time in the park if you want to. I was hapoy to have the large map of the park sold at the little shop near the ticket line. What I got on my entry ticket was so tiny as to be useless.

Don't take time away from Ljubljana. It is a charming city with actual sights to see, including a castle and the dragon bridge.

It's not Rovinj itself that needs more time; it's Istria as a whole. Porec is a very colorful coastal town (also very touristy; research the parking situation online before heading over). Vrsar is a cute, tiny village on a hill between Rovinj and Porec. The inland towns of Groznjan and Motovun are very atmospheric. Pula (where I've never been) has some Roman ruins; it doesn't draw overwhelmingly positive reviews.