Is there a Hadith stating that a child's sneeze during someone's speech indicates the truthfulness of what they are saying?

The hadith "Whoever narrates a hadith and someone sneezes at it, then it is true" has a very weak chain of narration (isnad). Mu'awiyah ibn Yahya, Baqiyah ibn al-Walid, is a Mudallis (one who practices tadlis in narration). Abu Hatim al-Razi, al-Bayhaqi, and Ibn al-Jawzi have all ruled it as fabricated or denounced. As for "If one of you sneezes during a hadith, it was true," its chain of narration is very weak. And "The most truthful hadith is that at which one sneezes" also has a very weak chain of narration.

As for the text (matn) itself, it is false and denounced. Sneezing does not indicate the truthfulness of a hadith, because observation attests to its falsehood, and a lie performs its own work.

There is no known origin for the addition of a child or boy in the hadith.

In summary: The hadith is false in both its chain of narration and its text.