Dave,
We went with friends who had a 8 year old in tow, most days were spent around our pool, or on the beach. Both parties hired cars, which had air conditioning, great to tour the island, spending time on the beach. Yes it was hot, but we all coped.
But if you want to visit Rhodes Old Town, or wander the streets of Lindos in the Midday Sun then you will be asking for trouble. If we wanted to go out we would head out early, and arrive back at the pool for the afternoon. Even as Adults the heat gets to you.
Another problem we had was there huge Wasps, they would not leave you alone if you were eating, we did notice that after dark you could eat in peace. They also had a problem with mosquitos.
Do not let any of this put you off, we had a great two weeks, and the Island is fab. Its all about being sensible, I usually get bitten alive but took extra measures.
Hope this helps,
Kaz.