Here are a few useful things regarding outfits and props for your family session:

• It’s recommended for all session participants to wear colors that match or complement each other. Best if you have no more than 3 colors overall, 4 max (can be different shades of one color, though).
• The best solution is to have neutral colors overall (white, grey, khaki, or jeans) and just add bright matching colors in accessories or shirts. It usually helps to start by picking out kid(s) outfit(s) and going from there.
• Avoid any logos or words on your shirts (unless all of you have shirts with cool family sayings, and that’s the look you are aiming for).
• It’s YOUR family session – let it express your personality! You can be as fancy (formal dresses) or as casual as you wish. 
• Professional makeup is recommended for all professional sessions for the best skin look. Removing blemishes is not included in general editing and is subject to retouching (extra fee). 
• If there is anything that has  sentimental value for you and you want it in the pictures, feel free to bring it for your session. Even better if it complements the colors of your clothes.
• If your session is in a park, it most likely has a grassy and sandy area and also uneven walkways. Make sure you are wearing comfortable shoes. 
• When it comes to the choice of shoes, the rule is the same as with clothes – best if it complements the overall style and colors. So either everybody wears sneakers, or everybody wears formal shoes, etc. 
• You might want to bring a color matching blanket or a cover to sit on for some pictures.

Below are the examples from some of my previous family sessions.