Need help with Family/Granddaughters portraits on beach

[theantiquetiger]

I am planning on doing a family portrait and portraits of granddaughters (no grandsons in our family). We are all going to wear similar clothing (white shirts, khaki shorts). I figure there will be a set of each granddaughter (5), each family (3), all granddaughters together, and full family (12 people). I know to shoot in the evening with sun on the right or left sides of the subjects. I plan to use off camera flash, camera flash, and/or white card reflector.

BTW, the beach runs east - west

What lens should I use?:

Tamron 70-200 2.8
Tamron 17-50 2.8
Canon 18-135 3.0
Canon 50mm 1.4

What will be my best distance from the subjects and best aperture setting for each lens above (I plan to shoot in AV)?

Any helpful hints will be greatly appreciated!!!

We are doing it this Thursday and Friday