Croatia is a trip in itself. Fly in to Trieste in italy. Drive to Slovenia you could go to lake bled first. Its not too far from Ljubljana so go there too.
Slovenia borders the istrian region of Croatia. There a lovely vineyards where you can get fab red wine in a plastic bottle just refill from the barrel. Great when your toddler is in bed.
There are some beautiful villages in inland istria. Coastal i would go to rovinj definitely then along to the canal or river that goes inland past there (seaside resorts of umaq, novigrad, porec, vrsar, etc might be worth stopping to eat lovely seafood and there are nice play parks in Croatia. Better than many other European countries for those kind of kid friendly facilities.i particularly liked the pirate ship one in vrsar. You can get boat trip up that canal from here too.
Pula worth a visit whilst in istria.
You should go inland to the piltivice lakes.
Zadar could be good. I dont know tge kavarna region.
The next good tourist thing i know before split is Trogir. Absolutely lovely.
Head towards Split. Its a really great city. Lots to see roman ruins great shops and restaurants really lively. For the dc there is a great park at the marina end of the city towards the big forest park on a hill.
From split there is krk national park which has beautiful waterfalls.
Heading down towards Dubrovnik is the highlight.
Eastern Europe is a huge area to cover. Budapest and Prague would be great. So is Vienna. Theyre more central than eastern.
In poland you coukd go to Krakow and warsaw and maybe wroclaw, Gdansk looks good too. Ive got my eye on Sopot near Gdansk as a seaside resort to take the dc to. Its meant to be very child friendly.
In Romania i only went to Bucharest and Brasov and they didnt make much of an impression. I liked the dracula castle area of Romania. That was different. Forget what its called.
Have fun planning your trip. Ive been all over eastern europe but not seen enough of it.