We went to Greece (Kefalonia) with our then 10 month old, at the end of May. VERY hot - he had a heat rash by the time we got out of the taxi from the airport! Rhodes is even further south and I'd expect it to be very hot in July.
Our DCs are older now, but still don't like excessive heat and we go to the Alps - still lovely and warm, but a milder climate and there is air! There can be swimming pools, bathing lakes (very clean and filtered) with 'beach' areas, picnic meadows, cycle paths etc - we love it