The longest beach in Greece by far is at Georgioupolis and stretches for miles on the Northern coast between Chania and Rethymnon. Plent of room to lose everybody.

please let us know where you will be in crete to tell you the nice beaches near by.
dont do it like a client of mine that was in agios nikolaos area and wanted to go for a day all the way to Grambousa just coz he was told it was a nice beach.
That distance is a good 4 hours in normal driving and in summer it can be 5 due to traffic

We are not sure We have been told that the north of the island is the best, but as for accommodation and resort, not decided yet!

Anywhere near Chania is fine, further east are Stalis which has a very nice beach with shallow water. Star beach in Hersonissos is small, busy, but a regular little sun-trap. Avoid malia - it's manic.
Both are in an area with plenty of resaurants, tavernas and nightlife - but not too rowdy - apart from after 8.00 on Star Beach.

Since you are not sure about which part of crete i will reply just for the east as its easier for me.
Malia=Big no for me even if sandy.
Hersonisos=so so as some people from malia visit it
Stalis=Not bad.Not bad at all
Elound=Sandy in some parts could have been better
Istron=Good one, avoid August
Sitia (town)= sandy
Vai=nice one but not sandy
Palekastro=nice not all of it sandy. hardly any waves
makris gialos area= most of them sandy
ierapetra=sandy bit busy
east of ierapetra=most of them by 90% not sandy
matala=parts of it sandy bit bussy

Marathi Beach on the Akrotiri peninsular (nr chania - Direct Greece) golden sand, sheltered and safe- no big waves or strong undercurrant and simply stunning views of the White mountains

Falasarna in the west is not just the best beach on Crete, it's been voted the best in Europe!
Most things in Western Crete are better than in the East...

Elafonisos and Falasarna every time! Have to be dragged away to go home...