Neutral colors are always best. Try to avoid heavy patterns, or putting multiple family members in different patterns. Instead, go for a color family. For example, different shades of blue with some cream, tan or shades of gray. Try a light pink paired with a pair of cute brown boots.
I recommend a "dressed up casual look", But above all I want these pictures to feel like you, so think of it as your style just dressed up a little. And finally, my last tip is that you go ahead and get your hair & makeup professionally done on the day of the shoot. I know it's an extra cost and more to think about, but in my experience it makes a HUGE difference in how the pictures turn out/how confident you feel during the shoot.