For boys... thick sweaters! There's nothing I like more than wool, winter sweaters. Combine them with a shirt and hat or scarf.
For girls, a pair of Minnetonka shoes are a must, as well as a faux fur waistcoat, which girls can wear with a dress or a shirt. My favorite combination is a waistcoat with a dress and high boots. And, you can find everything on Zara Kids!