SB 10.45.37

dvijas tayos taṁ mahimānam adbhutaṁ
 saṁlokṣya rājann ati-mānusīṁ matim
sammantrya patnyā sa mahārṇave mṛtaṁ
 bālaṁ prabhāse varayāṁ babhūva ha
Word for word: 
dvijaḥ — the learned brāhmaṇa; tayoḥ — of the two of Them; tam — that; mahimānam — greatness; adbhutam — amazing; saṁlakṣya — observing well; rājan — O King; ati-mānuṣīm — beyond human capacity; matim — intelligence; sammantrya — after consulting; patnyā — with his wife; saḥ — he; mahā-arṇave — in the great ocean; mṛtam — who had died; bālam — his child; prabhāse — at the holy place Prabhāsa; varayām babhūva ha — he chose.
Translation: 
O King, the learned brāhmaṇa Sāndīpani carefully considered the two Lords’ glorious and amazing qualities and Their superhuman intelligence. Then, after consulting with his wife, he chose as his remuneration the return of his young son, who had died in the ocean at Prabhāsa.
Purport: 

According to Śrīla Viśvanātha Cakravartī, the child was captured by the conchshell demon while playing at the Mahā-śiva-kṣetra.