Who would have thought then, that that kid would one day become a world champion? No one would have thought that kid would one day become a world champion.

AS I KNOW, IN CONDITIONAL CLAUSE NEVER USE FUTURE TENSE SO "KID ONE DAY BECAME A WORLD CHAMPION"'.AND IF FIRST CLAUSE IN PAST TENSE THEN SECOND ONE ALSO IN PAST TENSE. AND MAY BE "KID WERE ONE DAY BECOME A WORLD CHAMPION" BUT I HAVE DOUBT IN THIS. THANK YOU

In both sentences you need to add an extra 'that'. For example, 'Who would have thought then, that that kid...'

"would one day become"