I have always like some sort of coordination for my family pictures. All one color looks sharp and I thing brings out the "faces" of the people in the picture more, instead of the clothes. Sometimes, I have the girls dress with one color and the boys with a contrasting color. Sometimes, mom and dad will have print and the kids a solid out of one of their prints. Other times it has been a "rainbow" from oldest to youngest. Another idea that I like is to have the parents in a darker tone than the kids, but all the same color.
With outdoor pictures, which I LOVE, keep in mind the background you will be using. If there are vibrant colors (leaves this time of year), tone it down in your clothing colors to give a little contrast. White actually looks very nice against the rainbow of leaf colors. If mostly bright yellows and light orange leaves, use a green or maroon. However, if you are not using the fall foliage and have a lightly colored background, then go for the more vibrant colored clothing.
Hope that helps give you some ideas.