'The little boy's big mouth' shouldn't be marked wrong.
"The big mouth of the little boy" ?
yeap both are good... maybe check this out also
Could big mouth be understood like in English, as someone who talks a lot or spills secrets?
It is used for someone who talks back a lot, no matter the consequences.
The large mouth of the small boy was marked wrong
The little boys big mouth? No! Hmmm
"The little boy's large mouth" is an alternative translation and should be accepted.
Or "The little boy's big mouth." sounds natural in English. Your answer is slightly askew in that order.