As far as outfits go I would just wearing something of a similar style (not cut but style like sporty, preppy, vintage, trendy, comfy, etc) in colors that are complementary - not the same. I recommend staying away from too many patterns or anything else that might look too dated a few years from now. Wear something that is a notch up from how you would normally dress – not is dressiness but in style (add fun shoes, a cute hat, or funky jewelry. Easy enough right! Mostly though you should just wear something that is “you” and that you feel great in. You may find some ideas here: http://pinterest.com/michartphoto/portraits-family-tree/.
As far as props go… a nice, colorful or meaningful blanket is always a good option – quilted ones are really cute. It would also be good to bring a couple of their favorite toys or maybe a new one they would really get excited about (don’t let the know you have the toy until the end though.). I think punch balloons or the big rainbow lollipops are really cute in photos as well.
The main thing I am going to ask you to do as a parent is to interact with your children. So think about things that you love doing with them that make you and them happy (tossing them in the air, playing peek a boo, singing the alphabet, playing tag, tickling them)… that’s when I get the good photos. If your children are older you might consider basing the photos around an activity like baking or sledding.