Hi Shoestring,
We go to Cy every July Aug Summer school Holidays, I have three boys now aged 16, 13, 11 we have been going for 6 years in the summer hols. Admittedly they are dark skinned as their father is Cypriot but we have never had a problem with the heat. They do tend to need more sleep than at home but I think that applies with any holiday as they are so much more active than at home.
Your kids at ages 15 and 11 are more or less adults they will cope with the heat as well as we do. If they are extremely fair instill in them the need for sunscreen and regular breaks from direct sun. Plenty of water to drink and perhaps a hat will help.
Also if you are renting a villa you will have the option to go indoors in the aircon to watch some tv, read a book and relax in the hottest part of the day.
Don't be afraid of the heat we just adapt and slow down, that is the key.
Enjoy your holiday
Ruth