If there are 7 of you, For Crete you would need to hire more than a driver, you'd need a driver with a 9-passenger Van. As for Chania, please be sure where your cruise ship will land -- will it be Souda Bay?? That is east of Chania quite a bit look at this map ...
Do you see Souda Bay?? Do you see the Seafront of Chania where it says "Old Venetian Harbor"? That's Old Town Chania, where the grownups will want to spend their time wandering the very atmospheric & picturesque town. If your 4 teens want to swim there, see THat little "hook thing" at the far left of Old Harbor? Just beyond is "Nea Chora Beach" (= New Town Beach"). Not fancy but right on the small roadway, water is clean, good for a swim, lined with cafes that serve pizza & other teen stuff.
Here's what you could do (ENLARGE MAP NOW )all 7 of you take the bus in to Chania to the Bus station, which is direcly South of the Old Harbor ... walk up to OLD HARBOR, and walk around LH side of it out to the Point... there is the NAVAL MUSEUM ... lots of history but ONE THING that teens will go for -- a full-scale working Model of an ancient Greek warship, with OARS... it is full seaworthy, was built for the 2008 Olympics in Greece. Then One of you could walk west with the teens along seafront for about exactly 1/2 mile and Voila! Beach. If you can trust them to be on their own, give one of them a phone and instruct them to phone you every hour on the hour, give them pizza money, then leave. The beach will be perfectly safe, and so will they, unless you have some crazies in the bunch. The adult could go back & sightsee with theother grownups & assign someone else to pick them up, or else (by phone) assign them to walk back the way they came at 4 pm or so.