June in Europe Ideas - vacation + staycation

Consider going north - Denmark, Belgium, Germany - all fit your brief.

Copenhagen, Antwerp or Cologne would make lovely bases and easy for day trips to castles, windmills, beaches, rivers etc.

Copenhagen in particular is very kid friendly. We stayed in the Nordhavn area in September 2022 and it was full of kids on bikes and scooters, and people pushing prams.

Well, there a for sure many options. As Croatia is on your mind, how about:

1st week travel from Dubrovnik to Istria peninsula

2nd week stay in Istria either at a sea side resort or find a private house with pool somewhere in the backcountry (check out booking or other platforms, there are many options).
E.g. in Pula on Verudela peninsula there are several hotels and apartments, a lot of sport and fun activities and a public bus line to Pula downtown.

3rd week how about heading north e.g., settle down for a week at a Carinthian lake in Austria (lakes Wörthersee, Ossiacher See or Millstätter See are beautiful). A lot of sports and fun activities available (e.g., Hochosterwitz Castle, Minimundus, Pyramidenkogel, Gerlitzen mountain, castle ruin Landskron).
Or you may think about a farm stay in Slovenia.

In general, for half-day tours (valid for Istria, Carinthia and Slovenia) a rental car would be beneficial to be more flexible.

You mentioned the beach. I lived in Germany for four years and the Middle East for five years and can say generally that beaches in Europe can be disappointing compared to California, Florida and the Caribbean.

There are exceptions, like some Greek Islands and the Baleric Islands of Spain. The South of France, Nice, Monaco, Cannes the beaches were disappointing. Also, much of what we tried on the Adriatic coast of Italy. Also, didn't find Costa del Sol that wonderful.

Yes, there are some good places, but my message is do your research and don't just count on any beach area in Europe to be nice.

We love Croatia and Portugal, but didn't find the beaches that great.

You seem to want a beach in the southern part of Europe, but reasonable certainty it won't be too hot.

Think about the Algarve in Portugal. It seems to fit all your criteria. Relatively affordable too.

Or think about a lake in the southern foothills of the Alps, Garda maybe.

Lake Constance area is very nice and central to a lot. I'd stay at the West end of the lake a bit out of the town of Konstanz near a strandbad and community center.

I'd also think about Colmar. There is a swimming lake with a big recreational artificial beach club, great food, Central to a lot of fun things to visit, very nice climate.

Another one, if you want an idyllic place in the mountains but warm enough that time of year. Tannheimtal in Austria not far from Munich. It has a couple of beautiful lakes, one with a big public pool and park right next to it. Dedicated bike trails across the valley bottom, and plenty of riding on traffic free fire roads in the hills. Excellent hiking with a good huts and lift system to range farther. Great playground and indoor climbing gym at the community center. Great balance of tourists amenities and working small agriculture. Very pretty. Very safe place to let kids go free range. My family has spent a couple of weeks there over the past few years, and in that time we only once encountered English speaking tourists (an older Aussie couple). But it's only half an hour to Fussen.

The Munich lakes district also would not be a bad call at all, lots of recreational focused lakeside villages.

Or more touristy, but pretty darn nice, the Salzkammergut. St.Gilgen has all the amenities you are looking for.

Lastly, no guarantee a Northern Europe seaside location won't be cool and/or even rainy in June. Probably not - odds favor okay weather - but those areas of Europe can do "June-uary" any given year.