All inclusive? Budget? And would your dc be happy without a beach if the hotel had a good swimming pool?

His belts at that age I'd go AI - Balearics if the likes of Greece / Greek islands too hot. Ibiza would be my choice.

AI used to make me cringe but it really is the easiest option.

What doesn’t your DH like about the beach?
Could you go somewhere without him!

Cyprus is always a winner for us although I wouldn't go after May or before September for the heat. You can get some great prices and all inclusive is so easy, I didn't like the idea of it until we actually went!

The problem is a lot of Europe will be hot, some of it very! How important is ‘not too hot’ in your criteria? If it is important then maybe switch sea for lakes and look at Slovenia, Switzerland, Austria and Germany (Slovenia being the cheapest) and swim in lakes, go cycling etc and find a hotel with its own pool too?

If your DH likes eating out and Culture perhaps a hotel on B&B basis or an apartment on a complex with a pool would be a better option than an AI which can decentivise you from getting out and about. We found apartments better when DD was smaller as there’s more space and if you have a fussy eater (our DD would only eat chips and bread in restaurants) you can balance that out a bit. For history, beautiful beaches, great food, lovely scenery and child friendly people I’d look at the Neopolitan Riviera.

Lake Garda?

I was going to suggest a lake of some kind. We are going to Northern Italy (Garda or nearby). I think with 3 younger children water safety might make things stressful though?
We camped at Bigsee in Germany a couple of years ago. You can camp on the waterside, it might be a bit cooler than Italy.
Tbh I would go for beaches and DH could suck it up... My DH isn't keen on beaches either but for younger kids they are perfect.

Apart from the heat bit, I'd recommend somewhere like Antibes. Lovely beaches in the vicinity and the train runs up and down the coast so easy to go for days out. From Nice you can get up into the mountains. Or Italian Rivera as pp said.

Lake Garda would work from a POV of no driving. You can fly to Verona and get the train to the south end of the lake, then take buses or lake ferries to get around. There is masses of accommodation of various kinds, Gardaland (theme park), and culture if your DH wants that in Verona, Vicenza and various Palladian villas (although those will probably require driving to get to). There are masses of restaurants, bars, etc in all the the lakeside towns and its very pretty. Just get somewhere air conditioned with a pool, in case it's hot.

A lot of Europe will be very hot in the summer - Scandinavia might work for climate.

We stayed here which would suit children that age - had lots of alpacas you could feed and walk, swimming pool, restaurant and a lovely caring owner and unlimited hot chocolate. alpaca.se/ You would need to drive a bit to get there though its around 1.5 hours from Stockholm and would need to combine with somewhere else, maybe Stockholm. Copenhagen is nice for that aged kids as well. We've done Finland and Norway too but Finland was very wildlife based and your youngest is probably too young for that - it was like brown bears in the wild but you had to sit still to watch them for a few hours.

Or maybe Austria in the mountains, maybe a Kinder Hotel. We stayed at one in Hintertux where there is snow all year round but that one is more for a long weekend than a summer holiday. Or an AI is the easy option with pools and all on site and just shut your DH in a room and leave him to moan to himself whilst you and the kids have fun. There's excursions on coaches from those usually to avoid the driving - mine loved waterparks and that's great for cooling down but 3 year old maybe too young.

Hintertux one www.booking.com/hotel/at/familien-sporthotel-hintertuxerhof.en-gb.html