When deciding what to wear for photos, pick a palette of 3 - 4 colors. Think in terms of tones: blue tones, earth tones, neutrals, pastels, etc. Everyone in the family can express their own sense of style AND the whole group will coordinate beautifully.
It's best to avoid "matchy-matchy" (e.g. everyone wear the same color top), instead COORDINATE within your chosen color tones / palette.
Layering (cardigan, shawl, scarf, etc) or accessories can make a huge difference in tying together color palettes and adding small pops of color.
Skip logos and character/branded items (e.g. Mickey Mouse or Nike swoosh t-shirt). They don't photograph well.
Mix mostly solids and some subtle patterns and textures. Patterns are great in small doses and add a lot to your photos.
Avoid excessive use of bold, large prints that can distract from the photo. You want your photos to be about you and not merely your outfits.
Avoid wearing all white or all black clothing. If everyone wears it, the photo may feel a bit dull and pop less.
When in doubt, opt for classic over ultra trendy (unless that's your vibe.)
Flowy shirts and dresses may be really comfortable or currently in style; however, they can hide your shape and be less flattering in photos. Look for clothes that are neither overly baggy nor too tight. Think about balance between your upper and lower half. If you have a flowing skirt, choose a fitted top or with a loose top, choose skinny jeans.
Remember the shoes! Shoes have the power to complete an outfit or... completely destroy it. Avoid bright white shoes, athletic sneakers and kids’ shoes with cartoon characters and sparkles. If you can’t find shoes you love, consider having the family go barefoot for the session (just bring shoes to walk around at the location!).
Keep your outfits simple and comfy. If the outfit makes you feel self-conscious, it will show in the photos.