Please read carefully as many devotees find it hard to understand the original childhood Krsna never leaves Goloka-Vrindavana in the spiritual world.

So, who is the Krsna who enters Vrindavana in the material world? 

Krsna is always the Supreme Personality of Godhead and cause of all causes and from Him all Visnu-tattva expansions originate. 

Krsna does NOT come from Visnu as many Hindus wrongly believe, Kṛṣṇa is the source of all incarnations and Balarama is His first expansion.

Balarama is the source of all Visnu-tattva expansions. He is Kṛṣṇa's first expansion and is non-different than Kṛṣṇa. All Visnu-tattva expansions in both the Vaikuṇṭha planets and the material world come from Balarāma. 

Srila Prabhupada - "Kṛṣṇa never leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His expansions. Vāsudeva is Kṛṣṇa's immediate expansion, so Vāsudeva is not different from Kṛṣṇa. It is to be understood that the Vāsudeva referred to in this verse of Bhagavad-gītā is Baladeva, or Balarāma, because Balarama is the original source of all incarnations and thus He is the sole source of Vāsudeva." (BG 10.37, Purport)

As said above, Balarama is Kṛṣṇa's first expansion and from Balarama all other Visnu-tattva expansions originate, including the Visnu-tattva expansion who plays the part of Krsna when Krsna enters the material creation and also Kills demons.

The original Krsna as His Vrindavana childhood form, never leaves Vṛndāvana in the the spiritual world and never does He kill demons there. 

The fact is, there are no demons in Goloka-Vrindavana in the spiritual world, they only exist in the facsimile Vrndavana in the material world.

Srila Prabhupada - "The original expansions from Baladeva (Balarama), are Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha. All of Them are viṣṇu-tattvas, or nondifferent Personalities of Godhead." (SB 3.1.34, Purport)

Srila Prabhupada - "The original Lord Kṛṣṇa NEVER leaves Goloka Vṛndāvana. All the plenary expansions are one and the same viṣṇu-tattva." (SB 3.1.34, Purport)

Krsna has 64 qualities all the time. 

All His Visnu-tattva expansions, including Balarama, only have 60 of those 64 qualities all the time, but can, on certain occasions and pastimes, if playing the part of Krsna in the material world, can temporarily manifest those extra 4 qualities.

Krsna is non-different from His Visnu-tattva expansions however, there are some differences. 

For example, the original child Krsna in Vrndavana in the spiritual world never kills demons.

The original Krsna never leaves Vṛndāvana in the spiritual world, and Radharani the gopis there NEVER feel separation from Kṛṣṇa there because He is always with them as NOD explains.

Only in the Vrindavana in the material world do the Gopis experience separation from Kṛṣṇa and witness Krsna killing demons.

Only the "Visnu-tattva expansion" playing the role of Krsna in the facsimile Vrndavana in the material creation, kills demons

So, the Visnu-tattva expansions have 60 of Krsna's 64 qualities, which is having 93.75% of Krsns's qualities.

Lord Siva (Siva-tattva) has 55 of Krsna's 64 qualities that mathematically means Siva has 85.938% of Krsna's qualities.

The jiva-tattva (jiva-souls) like us, and also include Lord Brahma, have 50 of Krsna's 64 qualities which means they have 78.125% of Krsna's qualities.**