We have been there twice, although we have not been to all the ones that are on your list.
Mykonos and Santorini (especially Santorini) are very touristy. If you are getting off of a cruise ship in Santorini - allow for time coming and going on the lift or walking. We've done both - the path is walkable and they number the steps there. I think it was over 14,000 but we did it with a 5 year old and a 10 year old. May not be advisable unless your kids are pretty tough.
We took transfers from the top of Santorini to various beaches around the island....Red Beach, Perissa, and Kamari. These are not your traditional Caribbean style beach stops so Google around first to see what you like.
Most people do Oia and Fira on Santorini for the blue dome pictures you see all over the internet.
We also did a boat tour to Rhodes and I found it to be pretty underrated. We enjoyed cliff jumping and a beach there. The tour was about 5 hours long.
