So, it's about time you got a real photograph made of yourself or family. Great! Some suggestions below will help you prepare.
Think long term. Thomas the Train Tee shirts might be the thing right now, but might not be in 20 years. Keep children's attire simple and most importantly, comfortable!
Please make sure clothing is clean and ironed! If little Johnny is a dirt magnet that means he's having a great time exploring, but bring a change of clothes for him.
Co-ordinate clothing. All 5 of you don't have to have the same sweater or white shirts and jeans! Pick a single color that looks best on you and build around it with colors in the same tonal range.
Grooming: A little makeup goes a long way! If guys need a shave make sure there is enough time for any redness and irritation to dissipate. Don't forget your nails, and sox/shoe choices! Boat shoes are always great, flip flops maybe for the beach. Sneakers are good for sports photos. Heels for more formal portraits.
For Outdoor Sessions: Dress according to the surroundings we've talked about: For outdoor sessions, mimicking Mother nature, just a little more muted. Solid colors (no striped or big logos!) Fall: Long pants and sleeves for cooler temps. Reds, greens, light oranges and yellows. Summer and beach sessions: Short sleeves might be ok but longer sleeves are best; choose a light fabric. Blues, tans, greens. Think about where the session will take place and what we can expect to see there. Please! No short shorts unless we've discussed before! Let's talk about it if you have any questions!
For Studio Sessions or sessions in your home. By their nature these sessions tend more to the more formal. Think of the decor where your wall art will be presented. Use neutral colors so that the framed photograph will transition over the years as your tastes evolve. Maybe a suit and tie for the gentlemen and a more formal dress for the ladies. If that's too formal a dress down would be appropriate too. But the same guidelines as above apply: Coordinating colors; no stripes or plaids (unless you'll be hanging the photograph at your hunting lodge!)
Pinterest has some fun ideas!
The most important thing is do not stress! Make sure everyone gets a good night sleep the night before.