This question seems to be incomplete. However, there is enough information to find the right answer.
Answer: The missing child was found by other boy.
Step-by-step explanation:
The past passive is formed as it follows:
was/were + past participle of the verb
"Found," in this example, is the past participle of the verb "find."
"Founded" is the past participle of the verb "found," which refers to the act of establishing something, like an organization, so it´s not fitting for this sentence.