When you're dressing your family to go out, you probably pick the cutest thing for each person and go! It's hard enough to keep boys' shoes on, the girls from touching their hair, and steal a moment to do your own makeup.
I can appreciate that dressing for a family portrait or family mini session is a daunting experience. I can't help you when they're eating chocolate after they're dressed up, (except that I can with Photoshop!) but I can help you put everyone together a bit. Choosing outfits that coordinate as a whole family will help make your photos look as put together as June Cleaver's (with or without the pearls.)
Here are two options (plus a Bonus, Black-Best Option) for Picture Perfect, Color Coordinated Family Outfits:
Option 1 - Mono-chromatic (fancy for "all one color") - This one is a no brainer. Pick one color, and everyone wears it. Jeans and black shirts, or white shirts, or whatever. This can look dated boring if everyone wears exactly the same thing. But it will look clean and polished if you mix up textures and styles while keeping the color the same. Another great way to make things pop is to choose one "accessory color" to go with your monochrome outfits. Like the turquoise necklaces in this otherwise all-black family portrait.
Option 2 - Pick a Star Outfit, then coordinate: This option takes a little more planning, probably by Mom. I always think Mom should get the star outfit, but it's often someone the partner or the youngest child. Either way, the star outfir is the one with the bold pattern and bright colors. The fuchsia floral maxi dress or pink plaid skirt is the star outfit. Then, all the kids and partners wear solid colors that are pulled from the star outfit's pattern.
Bonus Option 3 - For Black-Belted Fashionistas Only. If you know your color wheel and color theory, you can choose an analogous (similar colors, like blues and purples) or complimentary (opposite colors, like pink and green or blue and yellow) color scheme for family. Done well, this can look amazing. But if the colors are off a little, it can go real bad... like bad enough to land your portrait on Awkward Family Photos. None of us want that.