GearXS offers the Kids' Merino Wool Blend Thermal Socks 2-Pack in Grey for $12.99. Coupon code "8OFF" cuts it to $4.99. With free shipping, that's $7 under the lowest total price we could find for a similar pair. They're designed to fit shoe sizes 6 to 8.
Note: They're also available in men's and women's sizes for 99 cents more.