dad - the general advice above covers you pretty well. wash your hands. make sure your nails are clean and trimmed. clean up your shoes. check the bottoms of your pants for fraying or holes if you walk on them. For wardrobe, go for jeans or pants, tucked polo or dress shirt with a belt, or go casual with just a T-shirt or untucked polo, button-up short sleeve, etc. In general, whatever your wife tells you to wear. Again you don’t have to perfectly match the rest of the family, just wear something that coordinates.
mom - i've found that women generally know what to wear, but unless you are extremely thin you may want to wear something that covers your upper arms. no matter how toned i think my arms look in the gym, when i get them in pictures i'm always disappointed & regretting the sleeveless wardrobe choice. long or 3/4 length tops are very flattering. if you wear jewelry, be subtle, and pay attention to it twisting and turning during the shoot. i will try to help you catch that sort of thing, but the less you have to fidget with it to begin with, the better.
kids – same advice as above, but again, everything doesn't have to match - simply coordinate with parents’ outfits. If Dad’s in a T-shirt, don’t put the kids in dress shirts – make it make sense.