If you are flying into Heraklion airport then I would definately recommend Agios Nikolaos, Elounda is also lovely, but I think with two teenage girls Agios may suit you better.
Agios would definately tick all your boxes, Agios is a working Greek town with an all round year population so although it does attract lots of tourists it also has a lovely Greek feel about the place too....and you definately wont find large single sex groups or 18 - 30 type tourists either, just under an hours transfer time from the airport, a good selection of shops,cafes, tavernas and restaurants and a few late night bars too.
You dont say whether you are going to be hiring a car? if yu are then one of the best sandy beaches on the north coast of Crete is the beautiful Voulisima or Golden Beach is only 15 min drive from Agios. The town also has a good choice of four local town beaches, 3 of which are sandy :-)
If you like the look of Agios then I would be happy to give you more specific recommendations for apartments, hotels etc depending on your budget...have fun deciding :-)