These two images were taken within minutes of each other. Same light, location, and pose. Wardrobe can greatly affect the overall look of you session.
I recommend finding one piece that is a must-have for your session. Whether it's your outfit, or for the children. Once you find that, you can begin basing the rest of the outfits off of that. Patterns are great, however, you don't want a pattern that is super busy. Polka dots, florals, and stripes can photograph nicely. Patterns with large, chunky designs, or chevron, do not photograph nicely. Textures and layers are great to add to create a more interesting look. A great example - a cotton dress paired with dainty lace stockings and leather boots!
Colors are the most important factor. My favorite colors to photograph are:
Any bright colors will reflect onto the skin of whomever is around them, called color casting. Colors to avoid:
Shoes to avoid:
PUMPS / STILLETTOS
GLADIATOR / STRAPPY SANDALS
Lastly, consider the fit. Your children's clothes should be their exact size, or possibly a little snugger. For men, I recommend a slimmer jean or pant than what they may normally wear. Overly baggy pants, jeans, or cargo shorts are not recommended. For women, flowy dresses photograph better and allow for more posing options.