Q:

Why does Krishna suck his toe while lying on a Banyan leaf?

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?)

A:

To show how sweet his feet are to the devotees.
So, he is hinting us to catch hold and worship his feet.
Babies have a tendency to suck their toes. It is a tendency to discover more about new things. If we notice, they try to keep every new item they see to discover more about that item.
But Lord Krishna is unlike other babies. Even though it looks like a normal action done by babies, there would be a symbolism in depicting Lord Krishna sucking his toe.
The Balamukundashtakam depicts different phases of Lord Krishna's childhood and leelas performed by him.
In the first stanza, beauty of Lord's feet and lord's mouth as an infant is described. In the third and fourth stanzas, it describes beauty of Lord Krishna's body and face. Next stealing butter, Kaliya mardanam (dancing on the hoods of snake Kaliya) and leela of freeing Nalakubara and Manigriva is mentioned. Hence, the mention of Lord sucking his toe in the Balamukundahtakam is a means of describing the beauty of his lotus feet and his lotus like mouth.
However, in the Srimad Bhagavatam Canto 12 chapter 9, we find sage Markandeya looking at Lord Krishna chewing his toe lying on a banyan leaf. There is a symbolism to it.  Srila Vishvanatha Chakravarti Thakura interpreted this act of Krishna in his commentary Sārārtha darśini

The infant putting its foot into its mouth is by S'rîla Vis'vanâtha Cakravartî Thhâkura interpreted as the Lord saying, 'see how sweet my feet are to the taste of the devotee'.

