Find something that makes you feel great but also comfortable. For my sessions, you should be able to move freely without any 'malfunctions.'
You'll be running, sitting on ground, leaning forward and possibly climbing a rock. Long flowy dresses are the best.
Use Design-seeds.com to coordinate colors.
You can search by color using whatever you're starting with - your dress, kid's clothes, etc. It'll return many different color palettes of coordinating colors.
My style of photography is joyful, clean, and bright. A color palette of all jewel tones or dark colors doesn't work well with my images/locations.
Navy, black, dark purple (among others) are totally ok in small amounts.
Notice the balance of navy shirt with yellow shorts, and mom's dress is black, but also bright blue & yellow.
This can be tricky for large families. I have four boys and it would be easy to put them all in the same thing.
But you want to mix it up - not all the shorts should be dark. Not all the shirts should be plaid. Use some solid colors, some print. Add a pair of embroidered shorts. Put someone in a Polo or linen shirt.
Accessorize! Bring a hat, large necklace, cardigan, etc.
Shop at ONE store!
Places like Old Navy, Gap, H&M have collections each season that automatically coordinate. Easy peasy.
Mix it up with some neutral (white, khaki, navy) pieces you already have and you're good to go!