This is such a common question.  There are countless recommendations online from professional photographers about what to wear and what NOT to wear.  Here are a few recommendations I give my clients: 

Number 1- Wear what is comfortable for you, that makes you feel like the best you and that flatters your body.

2. Take into consideration your background/ venue-  If you will be outside in a park, where there are trees and lots of green, avoid greens, and browns, so you don't blend in to the background.  If you will be on a beach, go for brighter colors, or whites.  

3. Think about the season- Layers work best for fall, and winter, as do scarfs and hats.  In the spring and summer, focus on light, flowing dresses or shirts.  Richer colors in the fall and winter, and brighter or pastels in the spring and summer.

4. Avoid being too matchy-matchy!  Everyone wearing the same outfit or the exact same color is a very dated look, and doesn't show the personality of the family members.  Instead, select a color palette that is complimentary.  Everyone can wear their own version of the color, in a style that fits them. (as long as the overall feel is cohesive and one person isn't super casual while everyone else is formal for example)

5. Think about shoes and accessories.  These things will be seen, so they should fit the overall feel of the session.  Make sure accessories aren't too bold as they will distract from the subjects. 

6. Avoid logos or character prints.  Printed clothing is ok, as long as it's not too bright or too bold. All of these things can be visually distracting in the portraits.  

My favorite look is a put-together casual look.  Adults in neutrals and kids in something more colorful.  I think simple is better for little ones.  This is true for my newborn and children's sessions as well.  I love the look of a baby in nothing but a wrap or naked, over a fancy outfit that doesn't look natural, or comfortable.  

Truth be told, there is no right or wrong answer for what to wear for your session.  If you are in doubt about what to wear, it's always helpful to talk to your photographer about what you have in mind or to even take pictures of the combinations you are thinking about for a second opinion.