How old are the kids? Just so we know what activities might be appropriate.
And the rental car: it sounds like you are renting in Italy (Venice) and returning there. Just make sure the car is authorized to drive into Slovenia and Croatia.
You have four weeks to visit and the area you plan to visit is all good. We have not been to Croatia but the photos I have seen of Rovinj are compelling. We have been to Slovenia, and really enjoyed Piran on the coast (swimming in crystal clear Adriatic water) , Ljubljana, and Lake Bled.
Allow at least a week to ten days for the Dolomites. So much to see and do there, and very family-friendly.