The very famous imagery of a baby Krishna sucking his own toes is mentioned in various places.
The first paragraph of the Balamukundashtaka clearly depicts the scene:
Kararvinde na padarvindam
mukhar vinde vinve shayantam
vatasya patrasya pute shayanam
balam mukundam mansa smarami.
which roughly translates to, "I pray to Balamukunda, who keeps his lotus like feet, near his lotus like mouth, using his lotus like hand".
The 9th chapter of Srimad Bhagavatam, describes Sage Markandeya seeing Lord Krishna sleeping on a banyan leaf and later:
he saw how the infant with the graceful fingers of His two hands grabbed one of His lotus feet and placed it in His mouth
Both of them, however, fail to mention the reason why he is in such a position. Is there a particular reason as to why he places his feet near his mouth? (Is it just to symbolize that he is a child, or is there any other hidden meaning to it?)