Beth has given you excellent advice and routing simply and accurately. I have planned 13 trips of my own and have planned countless trips for others using this basic template. It works.
The one MUST is to depart ASAP (allowing a 2-hour layover) from ATH airport on soonest available fllight to island #1 ... otherwise you lose precious time. I do hope your arrival will be before noon May 30, because that gives you the most onward flight options. Do NOT delay! YOu are already very late in booking seats for busy months, and Saturday is the busiest arrival day. Don't waste time using a multi-airline search platform --- The 2 domestic airllines are Aegean/Olympic and Sky Express: here are the links: http://en.aegeanair.com/e-services/e-ticket/ and http://www.skyexpress.gr/ Be prepared; because you're late in booking, your parties may have to book on different planes or even the 2 airlines .. but no fret, they land typically 30 minutes apart. So if booking is tight, 4 of you might be on an Aegean landing at 2:30 and the other 4 landing at 3pm on Sky Express.
Once you get to Santorini, the next challenge will be getting to your hotels/ apartment/lodgings. Santorini never has enough taxis, and the airport is an absolute zoo. Your best bet is to book through a private transfer service that will handle whatever your need is... perhaps 2 cars w driver, perhaps a mini-bus w driver. The absolute Master of Santorini knowledge, "Brotherleelove" (he's been posting TOP info for over 20 years), now advises this, so do JUST as he says: " Contact Gabi Birbeck through her Facebook profile. She lives on the island and operates a transfer service that will do just what you require."
And listen to Beth; her first trip was with a family, mixed ages. Go to Naxos next, skip Mykonos. You do not have time. The ferry transport between Santorini & Naxos does not hve to be booked now. If you want to FLY all 8 of you back to Athens (45 mins vs 5 -6 hours on ferry), you will need to book those flights now ... the Naxos planes are small. After transport arrangements are successfully booked, you can come back to RS Forum for help on lodgings. It will be a challenge -- but not as big a challenge as transit. ... because one can always find some hotel, but one can not magically "find" a plane.
Good luck!
