Unknown to them (the cowherd boys), it was actually Aghasura, a demon who had appeared as a python. Krsna, knowing this, wanted to forbid His associates to enter the demon's mouth - SB 10.12.25

Expressions researched:
"Unknown to them, it was actually Aghāsura, a demon who had appeared as a python. Kṛṣṇa, knowing this, wanted to forbid His associates to enter the demon's mouth"

Srimad-Bhagavatam

SB Canto 10.1 to 10.13

The Supreme Personality of Godhead, Śrī Kṛṣṇa, who is situated as antaryāmī, the Supersoul, in the core of everyone's heart, heard the boys talking among themselves about the artificial python. Unknown to them, it was actually Aghāsura, a demon who had appeared as a python. Kṛṣṇa, knowing this, wanted to forbid His associates to enter the demon's mouth.

SB 10.12.25, Translation

The Supreme Personality of Godhead, Śrī Kṛṣṇa, who is situated as antaryāmī, the Supersoul, in the core of everyone's heart, heard the boys talking among themselves about the artificial python. Unknown to them, it was actually Aghāsura, a demon who had appeared as a python. Kṛṣṇa, knowing this, wanted to forbid His associates to enter the demon's mouth.