Sandy Hook is fine for families, however...
It is mostly adults.
There is only one bathroom/changing facility and it's a LONG walk back there from the beach.
There is a Ferry that runs from Midtown and Downtown Manhattan to Sandy Hook with bus service to Gunnison Beach. The Ferry is expensive though and I believe it only accepts cash. But it is a very pleasant trip.
If you drive, you must arrive very early to get a parking spot; they fill quickly on summer weekends. Or arrive late in the afternoon.