Anonymous wrote:You come up with the color scheme and then have the parents buy their kids' clothing.
This. The intent is nice, but please reconsider buying kids' clothing and expecting them all to turn up and wear it even for the short period of shooting a photo, much less during a party. Choose the palette, find lots of examples online and send those links to the parents. Be prepared for someone to turn up in something that doesn't fit the palette perfectly. Be fine with it.
If you buy new clothes: One kid will refuse to wear anything he decides is "scratchy." Another will (as you seem to anticipate already) hate hate hate dresses and insist she wear leggings but not that skirt you got to go over them, and the leggings you bought are too tight/too loose/feel weird. A toddler will run screaming away from a blazer and has a bright red, teary face in the long-awaited photo. And so on. You and the parents will end up burning expensive photo shoot time begging kids to wear stiff new outfits "just for a few minutes, please! To make grandma happy!"
Please let the parents pick the clothing and buy and bring it with them because they know their own kids' preferences and what they'll actually wear. They can also get clothing their kids might wear again later.