Last year we went to Senj, which is also in Croatia. This summer we chose a little bit distant destination, Pula. Senj was 5.5 hours travel by car (500km), while Pula is about 6.5 hours (600km), so it was a quite exhausting drive with a 3 and a 4.5 years old children.
Pula is a small city, but the biggest in the peninsula with nearly 60.000 inhabitants. The city is to be located on the south part of the Istrian peninsula, near to Italy. It wasn't hard to recognize the similarities with neighboring country. You can easily feel it from the air, the style of the streets and houses if you ever been Italy. And they have got a coliseum.
My 4.5 year old daughter enjoyed very much swimming in the pleasant seawater, while my son didn't like the salt in it. At the and of our vacation my daughter was able to swim without any assistance, so I'm very proud of her.
We cheated a little bit, because we spent most of our days near to a pools of the holiday resort. There we could easily look after the children, when we could read and rest a while. Moreover there were active programs at the poolside for kids, which they enjoyed very much. The last week of our vacation there were a 30 minutes Mini Disco for them, which was they favourite program.