Pick your color palette first
Choose a palette of 3-4 colors to start. Think about neutrals, earth tones, pastels, etc. Everyone can express their own sense of style within the color pallet and it will make for beautiful coordinated photos.
Think about complementary colors either in the pallet you're choosing or in the location you're choosing.
No Logos or Big Prints
You want the focus to be on your family not on a busy pattern
Opt for smaller patterns that blend in. You can mix patterns with solids. That creates great variety.
Comfort and confident
Pick something you and your family are going to be comfortable in.
You want to feel confident and one way to do that is to be comfortable. If the kids are uncomfortable in their clothing, that will show in the photos.