I always tell people when they are going for family pictures to look on TLC.com for Jon & Kate + 8 (now Kate + 8) family pics... they had a wonderful knack for coordinating - NOT matching - outfits. And they always used really vibrant, lovely colors. I am SOOOO tired of seeing khaki pants or jeans and white shirts in family portraits!!!!! No offense intended, but c'mon people! Is that the most creative we can get? Yes, it's great to keep it simple, no bold prints or obnoxious accessories, unless you are doing a playful shot with everyone having the accessory, but to go so 'Catholic school uniform' with the khaki/white ensemble makes my eyes bleed!!! lol Sorry to be so dramatic...
One thing I sometimes try to do, is say, if everyone in the family has gorgeous blue eyes, maybe choose a color that compliments the eyes, and work with that... or choose 2 complimenting colors, check the color wheel for this, and have your family in shirts of these 2 colors. Or, if your background is going to be primarily greenery, choose a complimenting color for your clothes (green's complimentary color is the red family.) You could all wear something red, maybe cranberry red? (I prefer that to fire engine red... just my opinion!)
For the beachy shots, maybe mom wear a long flowy skirt - they are popular now, so easy to find in just about any color, with dad and son in linen shorts or capris (sorry, I don't know the name of the roll-up kind of pant/short combo for guys/boys). It's especially cute if mom gets in the water and her skirt gets wet on the bottom... maybe squatting down looking at something in the water with son, and dad looking on... of course, you'd want to save these for last, but still makes a gorgeous photo.
You are obviously using a professional photographer, as you are doing on-site photos (yeah - so much nicer than studio shots, imho), so I am sure he/she will do wonderful shots... I do photography as a hobby... and portraiture is my passion, so if you want any more specific ideas, message me...