Italy to Croatia

There are different transport options, it all depends on where in Croatia you want to go. Generally, there are buses to Istria and Zagreb going from Trieste through Slovenia. If you want to go to places in the south like Split or Dubrovnik directly, it's worth looking into flights, as it will be quite a long ride from Bologna.

What about ferries?

Thanks for your replies. We need transport from Bologna and plan to pick up a rental car when we arrive in Croatia. The ferries from Italy would be fun but don't fit our itinerary. Probably will go to Zagreb as our kids may fly in there to join us. Busy and confusing but I'm sure we'll have a great time.
Thanks again for your help

If you want to go to Zagreb from Bologna, you can take the train to Trieste, then a bus to Ljubljana, then a train to Zagreb. Yeah, that's a lot of connections. There's probably a direct bus to Zagreb from Trieste to make it simpler, but the idea of a bus ride that long doesn't appeal to me personally.

The Slovenian transfer service GoOpti can probably reduce the travel time a bit, but that means riding for hours in a van instead of a bus. I think your best value might be train to Trieste, bus or GoOpti from Trieste to Zagreb.

It would make things easier if you could stop over a night in Trieste or Ljubljana. Ljubljana is a lovely town, one of my favorites in Europe. If you've never been, I don't think you'd be disappointed with a night there for a quick look around.