First things first!
I am a huge proponent of being comfortable! If you or your family are uneasy with what you're wearing, you won't enjoy the session experience or your photos!
Coordinating neutral outfits
I recommend choosing a style that you'd like your family portraits to reflect (e.g., classic, formal, preppy, bohemian, sporty, etc.).
Next, pick a color palette that compliments your family! I recommend choosing one or two neutrals like cream, khaki, black, or denim. Then, you can select a few colors or shades of a single color. You'll want to make sure everyone's outfits stick to those colors.
- Note: I recommend avoiding overly matching outfits (e.g., all white tops and jeans) and contrasting colors like green and orange/red, or yellows and purples.
Once you've picked the outfits, if you can, lay the clothes out together to see how they flow!
If something seems out of place, replace it with another coordinating piece, and most of all have fun!
Freestyle it
Another method is to pick out your favorite piece of clothing and look at its colors. Next, be a sleuth, and go to everyone's closet and see what you can find that has some of the same colors in it.
It's totally okay if there's one color in one kiddo's pants and another color in another kiddo's top. If both of those colors are in the top you chose for yourself, that's coordinated!
Lay it all out, and review your hard work!
For some ideas and other tips (just for fun), this is a great article: What to wear: 30 Style tips to help anyone look amazing in photos.