What do I wear?
Has to be one of the biggest questions that photographers are asked before photo shoots.
Here are some general tips to use as a guideline.
Color is a good thing!  It's also always a good idea with color to match your season & location colors. And to wear colors that go well together!
Patterns are a good thing, as long as you are not dressing in competing patterns.   In a group setting, I suggest you start with a colorful pattern piece of clothing you love for one person in your group & then pull colors out of that for the outfits for the remainder of your group in more solids.  Try to think of everyone's clothing as one big outfit, while you might not choose a pattern for anyone, you can achieve lots of color, without everyone being completely matching.  Dressing everyone in all the same color is not recommended.
Some cute ideas for boys:
An unadorned white t-shirt with a simple button down over it, buttoned or open with denim jeans or khakis.  The layered look with a short-sleeve and long sleeve on top of one another.
Plaid shorts with layered polo shirts.  (long pants are recommended for older boys & men)
Some Cute Ideas for girls:
A simple slip dress with cowboy boots.
A patterned skirt with a top of solid color.
A crazy outfit that she always puts on when asked to dress herself. 
A few other things to keep in mind:
Avoid words or big logos on clothing.  Your eye immediately jumps to them when looking at your images.
Don't forget accessories on women/girls.  Earrings, great necklaces, fabulous shoes, fun hats and scarves all work to enhance images.
Dads, do not wear white socks with darker pants and darker shoes.  I know, sorry to even say it, but check the socks before you leave the house.  Also, if you are a watch guy, you might want to take it off for the time we are together.  
One more note of advice for men, shorts don't photograph well.  
Remember the key is to blend, not contrast each other,  like with mom in a white shirt and dad in a black shirt.  If you are dressing siblings, dress one in a pattern and solid color top or bottom & then choose  another coordinating color from the pattern for each sibling.  Trust me, it will work!!
If you can't decide between something funky and something a little more traditional, bring a change of clothes in the car.  We can take a break halfway through your session and you can all change in the car.
If you need some "clothing inspiration" check out websites like:  Old Navy, GAP, Gymboree, Children's Place, Matilda Jane.  They have lots of combinations together that you could then find in their store.
My final and most important advice is to wear something you are comfortable in and feel good wearing!
And make sure everyone has napped and eaten before they come! (nothing ruins a great outfit like hungry, tired people!)