What is the best over-the-counter medication for eczema in children?

Olive oil. For our kids in alaska (where we know about eczema!), we just put a little olive oil in their bath water every day and that worked great.
Moisturizers. Anything over the counter that can help moisturize the skin will be beneficial. I personally like eucerin cream. Its available in large jars and if you "drip dry" and apply liberally, it will help seal in the moisture and help the skin heal.
Petrolleum jelly. The drawback is the 'greasy' nature of this; however it provides with best sealing to keep moisture and it is inexpensive. Avoid compound with too many ingredients and certainly those with fragrances.