So I get asked this question ALL the time by clients and there is one rule - whatever you want. Helpful, huh? What I mean is the very most important thing when choosing an outfit to be photographed in is to pick something that feels like you. You need to be comfortable and confident in your clothes or else it really will show in your images. Stay true to your own style and then you can implement some of these other tips that will help you look more pulled together and translate well in pictures.
Probably the hardest element in preparing for a photo shoot is picking clothing, even more so if you are planning outfits for an entire family. The key here is coordinating vs. matching. Matchy- Match outfits (everyone is white shirts and jeans or all black) is outdated and it will cause your images to look that way too. It also doesn't allow you to express any personality which is not what you want. The key to coordinating outfits is to pick one or two colors that look good together and then choose a complimentary color as an accent. Your accent color is typically brighter and adds a fun pop to your outfits. Don't be afraid to add patterns and textures in your outfits as well. Just make sure you don't overwhelm. It's a good idea to use pattern on one person if you choose to incorporate it and if you are still a little unsure use it on a child. They can get away with a little bit more as far as "craziness" in their clothing. Textures, either through layering clothing or accessories also make an image more interesting and adds depth. Think tights under dresses, layered sweaters or cardigans, a scarf, chunky necklace, ect. Accessories are also a great place to incorporate that accent color as they are less overwhelming.
Another thing to consider are jeans. We all love them and feel comfortable in them and hey they go with everything! But they can be a photo no-no if you are not careful. It's always nice to break up a sea of denim in pictures with a skirt, dress, or other colored pant. This keeps you from going back to that outdated everyone matches look, and again adds more interest to the image. If you are going to have multiple people in jeans, wear different washes and fits. If wearing skirts or dresses keep in mind your hem length. You want to be able to sit and stand in it and not regret it in 10 years. This is probably not your best bet as far as showing your wild side. Save that for accessories or prints. Also be mindful of logos, generally they are not a good idea in pictures. Once you have your picture taken in that logo you are married to it. Plus it can be distracting in the image. Have fun with kid's outfits. As I mentioned before this is usually a "safe" place to go a little crazy. I love kids in fun hats or rainboots for example. They can get away with it and it shows off their personality.
And a few final tips - make sure you (and children) are comfortable. If you can't move, you are seriously limited as far as posing and an uncomfortable kid is an unhappy kid. I am sure I don't need to tell you what an unhappy kid leads to! The last little tip I will leave you with is location. Consider your location when choosing your outfits. Does it work, does it make sense and will it be interesting? If you are being photographed in a park in the spring avoid green. You will blend in. I am currently in the process of finding outfits for my boys' bluebonnet pictures. I am avoiding blue, or at least bluebonnet blue. Navy and teal and other variations are ok but blue on blue is a lot of blue plus it adds no visual interest to the scene. If you are using props consider this as well. Putting a baby all in red in a red wagon isn't going to look great, neither is sitting on a pink chair in an orange dress.
If you are unsure how something is going to look ask! And remember that pictures are an investment so not only do you want to look nice but most importantly you want to look like you!
Here are a few examples of what I am talking about and why it works...
The main color pallet here is black and grey, which is more neutral and traditional but the accent color of hot pink makes it really pop and more playful. Notice how the girls are wearing the pink and parents are more neutral. This is exactly what I am talking about when I say that kids are a "safe" place to incorporate riskier elements. These girls totally pull it off! There is texture in the older daughter's shirt and in the layering of the son's sweater. They avoided the denim trap by putting the girls in tights and colored pants. They don't match the background but they look natural in it (like this is something you might actually wear to a park in the fall).

Here is another example. The main color here is blue which is a little less traditional but still neutral enough that they won't regret it later and mom added a fun accent color in the orange of her scarf (which also adds texture to her outfit so bonus points!). They have a good variety of blues (teal, navy, royal) and patterns -stripes in dad's shirt and plaid in the little brother's. The fact that they are both blue tones keeps the two patterns from competing. All three guys have different wash jeans and mom breaks it up with her teal pants. Again they look natural and comfortable in what they have on.

Ok, last one. The color pallet here is mostly grey and denim with a pop of red. The kids have on the brighter red and they add texture and pattern in the plaid shirt and material of the girl's dress. Even though mom and the boy both have jeans they are totally different colors and the girl's dress adds variety. They coordinate great with the background (which can be trickier in an urban setting), while they are wearing some of the same tones as the wall and brick behind them they don't blend into it thanks to the red and plaid.
Hopefully that helps, like I said picking clothing for pictures can be stressful! Stay tuned for more in this "Preparing For Your Shoot" series, including a guest post from my makeup artist friend over at Mandy Marie Makeup with tips for photo ready makeup!