Coordinating colors not matching
Matching works, but only if everyone is consistent.  Often the lack of contrast can make everyone blend into one another.  Consider a couple colors that will fit into a color scheme.     

Look at your home decor.
These photos will want to hang on the walls of you home.  What's your decor?  Bright colors?  Neutrals? Consider your colors in relationship to you home decor.

Don't forget accessories
Accessories not only add to your outfit, but they can offer a little more fun in photos with little ones who wish to tug on ties or admire mom's large necklace. 

Consider not having characters
I know our little ones love their characters, but characters on a shirt can distract away from our children's expression.   

Textures are your friend
The Textures, scarves and belts add a little more to the image

Avoid all white or all black clothing
If you're wearing all white it can easily get overexposed compared to other elements on your attire.  Black is to easy to underexpose and loose detail. 

Think classic
These photos will hang on your walls for years.  By choosing simple, classic clothes you will help give you images a timeless feel.