Who is a Sannyasi and how do the scriptures describe an ideal Sannyasi can be read here
Why do we cremate?
Hindus traditionally cremate their dead because a fiery dissolution of the body brings swifter, more complete release of the soul than burial, which preserves the soul's psychic connection to its just-ended earthly life. After death, the departed soul hovers close to the earth plane in its astral body, emotionally attached to the physical body and its old surroundings, still able to see this material world. The funeral rites and burning of the body signify spiritual release, notifying the soul that, in fact, death has come. Some of the funeral chants address the deceased, urging the soul to relinquish attachments and continue its spiritual journey. The Gods and devas are invoked to assist the soul in its transition. The fire severs ties to earthly life and gives momentum to the soul, granting at least momentary access to refined, heavenly realms. All attention is on a singular goal, as expressed in this prayer from the Rig Veda:
"Release him again, O Agni, to the fathers. The one offered to you now
proceeds to his destiny. Putting on new life, let him approach the
surviving, let him reunite with a [new] body, All-Knowing One!" (10.
So a person is cremated for his soul to achieve after death what a sannyasi has already achieved before death.
The sannyasin has no sacrament involving the sacred fire: he has the
fire of knowledge (jnanagni)in him. His body is not cremated - that is
there is no Agni-samskara for it- but interred as a matter of respect.
Strictly speaking, it must be cut into four parts and consigned to the
four quarters of a forest. There it will be food for birds and beasts.
In an inhabited place the severed parts of the body would cause
inconvenience to people. That is why they were thrown into the forest.
There it would be food for its denizens; if buried it would be manure
for the plants. Now over the site of the interment of a sannyasin's
body a Brindavana is grown [or built] : this again is done out of
respect. At such sites all that is to be done is to plant a bilva or
When the Yogi separates himself from the physical body at the time of
death, Brahmarandhra bursts open and Prana comes out through this
opening (Kapala Moksha). “A hundred and one are the nerves of the
heart. Of them one (Sushumna) has gone out piercing the head; going up
through it, one attains immortality” (Kathopanishad).
To attain this or symbolically yogis and sanyasins after death are placed in yoga posture. Sometimes kapala moksha is made by throwing a coconut on the head. Then the mortal body is buried.
Brahmarandhra: When the Yogi separates himself from the physical
body at the time of death, this Brahmarandhra bursts open and Prana
comes out through this opening (Kapala Moksha). “A hundred and one are
the nerves of the heart. Of them one (Sushumna) has gone out piercing
the head; going up through it, one attains immortality”
Why bury children?
Cremation is the norm in Hinduism to dispose of the dead body. However, this norm does not apply to children. The bodies of young children who die within a year from the date of their birth are usually buried rather than cremated. In some instances they are consigned to rivers. In other words their bodies are offered to water or the earth, rather than fire. It is because the subtle bodies of children remain underdeveloped and hence unfit for fire sacrifice.
The final rites of a burial, in case of untimely death of a child, is rooted in Rig Veda's section 10.18, where the hymns mourn the death of the child, praying to deity Mrityu to "neither harm our girls nor our boys", and pleads the earth to cover, protect the deceased child as a soft wool.
Betake thee to the Iap of Earth the Mother, of Earth far-spreading,
very kind and gracious. Young Dame, wool-soft unto the guerdongiver,
may she preserve thee from Destruction's bosom.