Here are some ideas from my research on Pinterest.
If you are having an outdoor photo shoot avoid going too dressy. It usually does not blend well with a park type setting. Also do not forget the time of year. It is hard to get a kid to smile in a sleeveless dress in 50 degree weather. Lastly, do not forget about time of day. If you have a morning shoot the light will be lovely, but there will be dew on the ground. Wearing a white skirt is probably not the best idea, unless you are go with the flow and understand you will be visibly dirty after sitting on wet grass.
Choose a neutral base color pallet with pops of color. I usually tell people to pick one neutral color such as navy blue, brown, white or grey. I try to avoid black. If it is a fall photo shoot try for navy blue with pops of orange or green. One of my favorites is a brown/cream base with pops of maroon for fall. You can certainly use more then one pop of color. If you go back to the navy blue base you could do pops of green/pink. The key to making this idea look natural is not everyone should be wearing the pop of color. Another tip to making it look natural is to mix up the types of clothing everyone is wearing. So if you have a dad and son both wearing white dress shirts put one in a brown corduroy jacket or a vest to help mix things up a bit. Everyone wearing a camo jumpsuit is not coordinated, it is crazy! So do not worry about everyone matching perfectly and just keep the outfits in the color scheme.
Be careful with loud patterns and stripes. As a general rule patterns and stripes can bunch up in odd ways and not photograph as well as solid colors. So if you are going to wear something with a pattern, snap a quick picture of yourself both standing and sitting to see if this pattern will work.