WHAT TO WEAR
Keep your clothing simple, flattering, and coordinating (but they don't have to match perfectly!).
The most important aspect of a wardrobe is comfort and liking your outfit -- especially for mom!
If you like your clothes and how you look in them, that will show in your pictures!
PATTERNS/STRIPES: If you like stripes or patterns, keep them BIG. Small stripes and tiny patters can produce an effect that just looks weird.
FABRIC CHOICE: Jeans or sturdy shorts/capris are the timeless choice and they're great for fun, casual pictures because they are flattering and don't show wrinkles or panty lines. Avoid linen or fabric that will get too "clingy" and see-through if it gets wet if you're having a beach session!
COLORS: Lighter shirts/tops give the pictures a more casual and bright look. If everyone is in a darker shirt, that will give the portraits a more serious and thoughtful feel.
"MATCHY MATCHY?": Not everyone has to wear the same color! You can match if you like, but I recommend avoiding completely matching colors. I would ask you, "Do you walk down the street together wearing jeans and white shirts?" Coordinating colors are visually pleasing and add interest and variety without being too busy or too boring.
LITTLE ONES: For children, choose outfits that fit well without a lot of "growing room". For infants who not standing on their own yet, try to avoid overall-type outfits since the bib of the overalls always ends up shifting up and covering your baby's adorable face.
SNEAKY TIP: One of my favorite tricks is to Google "What to wear photoshoot" and look at the IMAGE results.
Let your mind drift over all of the pictures there until you land on something that catches your eye, and you're on your way to creating a great color combination that you'll love!