It is often difficult to know what to wear to be photographed. View my Pinterest board at Kristin Smith Photography "What to Wear for Group Photos" for great ideas.
Coordinate colors but do not match. Pick 3 colors, vary the tone, and follow that scheme in each outfit. Using colors from your home will result in a beautiful wall presentation. Layers and texture in clothing add a lot of interest. Many families prefer to choose the mother's clothing first.
Accessories add to the outfit and also perform well as a prop during the session.
Tops with sleeves past the elbows are most flattering. Loose flowing clothing can add weight to a person as the body form is not defined. Think of about the silhouette of the outfit and wear clothes that show form, but not too tight. Bright/light clothing can call attention to an area whereas dark clothing can be slimming and allow the attention to be on the face. Think long vertical lines. Wearing high heel shoes/boots with long pants will add height.
If you are going for a trendy look use patterns and layers. Otherwise, limit the number of patterns as it can be distracting when there are several people wearing a variety of bold patterns. Do not wear clothing with distracting characters.