Thanks for posting your entire itinerary. That is very helpful. I tend to think more in terms of nights than days when I'm planning. If I a am reading your itinerary correctly, your original plan is...
- Arrive Zagreb
- Llubljana x 1 night
- Julian Alps x 2 nights
- Rovinj x 1 night
- Pula x 1 night
- Zagreb x 1 night
- Plitvice x 1 night
- Split x 3 nights
- Hvar x 2 nights
- Dubrovnik x 4 nights
- Depart Dubrovnik
So... you have 6 nights before arriving at Plitvice
Thought #1: All those one-night stops look a little brutal at the beginning of the trip. There is one guy on the forum who says his family loves to travel this way, but most of us would find that much packing and unpacking miserable and very costly with respect to time.
Thought #2: Is there a reason you are returning to Zagreb mid-trip? That seems like a big detour in your original plan -- it turns a 3-hour journey from Istria to Plitvice into a 5-hour journey and adds a travel day. if Zagreb is on the list of places to see, I would stay there the night your arrive in Europe and avoid back-tracking there.
Thought #3: If you decide to stay in Zagreb the first night, you have 5 nights left until Plitvice. You can do the standard US traveler "greatest hits" trek through Slovenia in 3 days (Ljubljana, Bled, Postonja/Predjama) + Croatian Istria in 2 days. As others noted, the "greatest hits" option is likely best accomplished through a single lodging locale -- Ljubljana. You could explore Ljubljana on arrival day, do a day trip to Bled/Bohinj (leave early so that you can get to Vintgar Gorge by 8 am), have a free day for a not-so-US-American destination (Velika Planina + Kamnik?), and then do a cave on the way to Istria. I would be inclined to do Pula as a day trip from Rovinj to save another day of changing lodging.
Thought #4: For caves, you have 2 choices -- the theme-parkish Postonja Cave or the "true nature" Škocjan Caves. Slovenians like nature, so all the ones with whom I talked during a 25-day trip through the western half of the country recommended the latter, which is what I did. I thought it was amazing. The scene from one of the Lords of the Rings movies where Gandolf says, "You shall not pass!" kept flashing through my head. Your choice may depend on the condition of your senior traveler.
Thought #5: You could use all 5 of the remaining days between Zagreb and Plitvice for Slovenia, which would make 2 stops in Slovenia a little more manageable. This opens up a lot of options. To narrow them down, it would be helpful for you to answer the question that Emily hints at... What do you want to get out of your time in Slovenia? Ljubljana? Mountains? Hiking? Rafting (as may be implied from another post to which you responded)? If rafting is on your list, you want to spend some time on the western side of the Julian Alps. Tourist Farm Kranj just above Drežnica (which is near Kobarid) has a couple of family rooms that would meet your family's needs and give easy access to that side of the Alps.