|Unlock Forum posting with Annual Membership.|
This is a section from the help center on what is postable or not postable. The question about children's books is at the bottom.
Pamphlets/Booklets/leaflets without ISBNs: NO
Media Mail doesn't have a minimum size or weight, but there is a thickness minimum for Delivery Confirmation, 3/4". The weight limit for MM is only at the top end, max 70 pounds.
There's this: Books, including books issued to supplement other books, of at least eight printed pages http://pe.usps.gov/text/dmm300/373.htm#wp1015205