"Once, when Krsna and Balarama were playing on the bank of the Yamuna, a demon of the name Vatsasura assumed the shape of a calf and came there intending to kill the brothers. By taking the shape of a calf, the demon could mingle with the other calves. Krsna, however, specifically noticed this, and He immediately told Balarama about the entrance of the demon. Both brothers then followed him and sneaked up upon him. Krsna caught hold of the demon-calf by the two hind legs and tail, whipped him around very forcibly and threw him up into a tree. The demon lost his life and fell down from the top of the tree to the ground. When the demon lay dead on the ground, all the playmates of Krsna congratulated Him, "Well done, well done," and the demigods in the sky began to shower flowers with great satisfaction. In this way, the maintainers of the complete creation, Krsna and Balarama, used to take care of the calves in the morning every day, and thus They enjoyed Their childhood pastimes as cowherd boys in Vrndavana."
Krsna Book, Chapter 11
"In Vrndavana, both Krsna and Balarama, after finishing their childhood pastimes, began to take charge of the calves and send them to the pasturing grounds (go-carana). During this time, a demon named Vatsasura entered among the calves and was killed, and another asura, in the shape of a big duck, was also killed. The playmates of Krsna narrated all these stories to their mothers. The mothers could not believe their children, Krsna's playmates, but because of full affection they enjoyed these narrations of Krsna's activities."
Bhaktivedanta Book Trust. Excerpted from text and purport of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.