Should a discerning child pray next to the men in congregational prayer, or should he be positioned at the back? Is there any evidence for this?

There is no harm for a discerning child who maintains his purity and prayers, and does not disturb the worshippers, to pray in the middle of the men's rows. This is supported by the hadith (definition) of Ibn Abbas regarding his prayer in Mina while he was a young boy. It is also permissible to designate a row for children behind the men, especially if they are numerous and do not cause disturbance. The hadith "Let those of you who are mature and discerning stand close to me" is understood as an encouragement for the wise and discerning to be early and close to the Imam due to his need for them, not as an encroachment on the place of those who arrived first.