CC Ādi 5.138

vṛṣāyamāṇau nardantau
yuyudhāte parasparam
anukṛtya rutair jantūṁś
ceratuḥ prākṛtau yathā
Word for word: 
vṛṣāyamāṇau — becoming like bulls; nardantau — making roaring sounds; yuyudhāte — both used to fight; parasparam — each other; anukṛtya — imitating; rutaiḥ — with cries; jantūn — the animals; ceratuḥ — used to play; prākṛtau — ordinary boys; yathā — just like.
“Acting just like ordinary boys, They played like roaring bulls as They fought each other, and They imitated the calls of various animals.”

This and the following quotation are from the Bhāgavatam (10.11.40 and 10.15.14).