Chilly and Milly are Frosty and Crystal's twins who appear in Rudolph and Frosty's Christmas in July. It is never explained how or when Frosty and Crystal gave birth to Chilly and Milly (though they were likely built out of snow just like their parents). They prove to be just as vulnerable to warm weather as their parents. They also love Rudolph very much, looking up to him as their adoptive uncle.
Sometime after Chilly and Milly were created and brought to life, Rudolph is invited to help a struggling circus for an old friend, Milton the ice cream man. The wicked wizard of the North, Winterbolt, strikes a deal to give enchanted medallions to Frosty's family for the duration of the circus that will protect them from the heat; however, once the hundredth firework for the closing Fourth of July celebration goes off, Frosty's family will be vulnerable to melting again. To protect them, Rudolph accepts blame for the theft of the box office earnings, breaking Chilly and Milly's hearts at how Rudolph has seemingly become a criminal. Later, when Frosty surrenders his hat to Winterbolt, Chilly and Milly are devastated over their father becoming inanimate again.
When Rudolph returns with Frosty's hat, and also returning the box office earnings and revealing the true culprit to be Sam Spangles, Frosty is restored to his loving, living snowman self. Unfortunately, when Winterbolt's Ice Scepter is shattered by Lilly Loraine tossing her guns at it, causing all of his magic to be undone, it causes Frosty, Crystal, Chilly, and Milly to melt, leaving everyone upset over how they are going to revive them with a December wind when it is July. Thankfully, Big Ben, one of Rudolph's companions from Rudolph's Shiny New Year, arrives with Jack Frost, who uses his winter wind to restore them back to their normal selves, and maintain them until they can be returned safely to the North Pole by Santa and Mrs. Claus. They wave good-bye to Rudolph as he leads the now airborne circus on their way out of their financial problems.