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.
Translation: 
“Acting just like ordinary boys, They played like roaring bulls as They fought each other, and They imitated the calls of various animals.”
Purport: 

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