What Makes a Children's Book a "Classic"?

I think the answer is quite simple.  Will your children choose to read it to their children, and so on?  It isn't the critic's reviews.
The Children's Room has been very fortunate to receive a wonderful collection of "classics".  Come in and discover if they might become classics in your family.
Here are a few examples:
Bark, George by Jules Feiffer
Frederick by Leo Lionni
Mr. Popper's Penguins by Richard and Florence Atwater