Vienna-Slovenia-Venice

Sounds like a jam packed itinerary for ten days. I don't see how you can see it all in such a short time. If you pick up a rental car in one country and return it in another, there is a large drop off fee. We were quoted 1000€ for picking up in Lisbon, dropping off in Barcelona. So, we dropped car off in original country, Portugal, and took taxi across border and rented another car.
In Vienna, Schonbrunn Palace has a zoo in it's park. Many places you plan to visit would not appeal to a young child such as Schonbrunn Palace, Peggy Guggenheim Museum,Kunsthistoriches Museum. Perhaps you'll take turns visiting them so the child doesn't have to tag along. They are all excellent for adults, however.

Car does you no good in Vienna. Skip Graz and take the morning direct EC train to Maribor and rent a car there for your time in Slovenia. Don't know if you can get through your whole list in 4 days in Slovenia, so decide what your priorities are. See if you can drop the car in Nova Gorica on the Italian border. Skip Trieste and take a train to Venice from Gorizia.
So that is 3 days in Vienna, 4 days in Slovenia, 3 days in Venice.

Previous suggestions re car rental are recom for sure. I have been to the countries u mention and I believe your scheduled itin is too ambitious.
Pare it down to your must dos .

Don't forget that you are travelling with a small child. If you are loving parents, you should show some consideration for your kid.

You itinerary is absolutely over-the-top, even if there were adults, only.

My advice: Spend 5 days in Vienna, and 5 days in Venice.

Getting 5 people up and out of the hotel each morning will not be a quick process, so you are not going to have time for all the things uku want to see.

Leaving Slovenia for another trip isn't a bad idea, though I suspect there's more there to interest a child than on your list of Viennese sights. If you do go to Slovemia, pick just one of the cave systems. Maybe Postojna since you have a 5-year-old and there's a little train that covers part of the distance. I think one Slovenian castle would be enough. Don't miss the dragon bridge in downtown Ljubljana.

Just be aware that there's an international rowing competition at Lake Bled from September 6 - 10. It won't be the peaceful lake it normally is from Wednesday - Thursday! It will be fun for the rowers (me!!) but I feel a little bad for the people who may not be aware and just happen to come during the regatta.

That's a great idea to fly in to Vienna and out of Venice because there are numerous options for covering what you'd like to see.

I know these areas rather well, and my guess is that hitting all those destinations would require a LOT of time in the car, much zig-zagging, and less enjoyment than seeing few sights with more time at each...especially since one of your travelers is but five years old. :)

If I were doing this trip, I would edit out many of the destinations listed and break up the journey like this--with the number of nights in each place depending on how much time I wanted to spend at each place:

-Vienna to Lake Bled
-Lake Bled to Piran (you could also route that so that you stop in Ljubljana for several hours along the way)
-Piran to Venice

By the way, in my experience, Google map estimates seem to be based on breaking the speed limits, never stopping, no traffic especially at the A/SLO border, no snow delays, and bionic superpowers, haha! :) In short, I'd tack on an extra 2-3 hours on to what Google maps out per route, time-wise

What I might also consider is skipping Slovenia and the rental car altogether and remember there is direct night train service between Vienna and Venice, and I'd do half my trip in each city. The "Euro Night" (EN) train is convenient (see the Austrian National Railways page www.oebb.at for schedules, etc.), easy, and you can get a sleeper car with breakfast. There are plenty of reasonable hotels near the train stations in both cities, too.

And at that time of year, Christmas markets will be up and running as well.

Happy planning!

Suggest you take the train to Bled from Vienna and you can rent from Hertz at the station, and drop the car in Kopar which is outside of Piran; from Hertz you can walk to the bus station that takes you to the Trieste train station where you can ride to Venice.
We did this basically last month, the only difference is we didn't know about the agency in Bled, so we had to pick up the car at the Ljubljana airport and back track to Bled.
The rental car was really cheap-4 days, super CDW and fuel option (€66) totaled €166, for a 4 door VW(small!) which was fine for the two of us with luggage. We stayed in Piran, and 24 hour parking at the garage outside the city was €17.
We will be back next year for at least a week to cover what we missed this time.

simisalim, our family will be following right behind you and we'll be following that exact route middle of Sept. The train from Vienna to Bled is nearly a full-day but, we figured taking the train would reduce the stress level of route navigation, while being able to enjoy the sights from our train seats. From Ljubljana, we decided to take a GoOpti van to Venice, a bit faster than taking the train. The beauty of Ljubljana is it's outdoor scene, soak-it in and enjoy.
You've got quite a bit packed into your 'visit wish list', figure one major site per day, and mix-in some casual day's so everyone doesn't get burnt-out. After 2-3 churches, castles and museums, you're not gonna want to see another as they all start looking the same.