SB 10.13.18

tataḥ kṛṣṇo mudaṁ kartuṁ
 tan-mātṝṇāṁ ca kasya ca
ubhayāyitam ātmānaṁ
 cakre viśva-kṛd īśvaraḥ
Synonyms: 
tataḥ — thereafter; kṛṣṇaḥ — the Supreme Personality of Godhead; mudam — pleasure; kartum — to create; tat-mātṝṇām ca — of the mothers of the cowherd boys and calves; kasya ca — and (the pleasure) of Brahmā; ubhayāyitam — expansion, both as the calves and as the cowherd boys; ātmānam — Himself; cakre — did; viśva-kṛt īśvaraḥ — it was not difficult for Him, for He is the creator of the whole cosmic manifestation.
Translation: 
Thereafter, just to create pleasure both for Brahmā and for the mothers of the calves and cowherd boys, Kṛṣṇa, the creator of the entire cosmic manifestation, expanded Himself as calves and boys.
Purport: 

Although Brahmā was already entangled in bewilderment, he wanted to show his power to the cowherd boys; but after he took away the boys and their calves and returned to his abode, Kṛṣṇa created further astonishment for Brahmā, and for the mothers of the boys, by establishing the lunch pastimes in the forest again and replacing all the calves and boys, just as they had appeared before. According to the Vedas, ekaṁ bahu syām: the Personality of Godhead can become many, many millions upon millions of calves and cowherd boys, as He did to bewilder Brahmā more and more.