Wa'alaykumsalaam Wa Rahmatullah Wa Rahmatullah
If a child who hasn't reached the age of puberty, he may lead the prayer as long as his Tajweed of the Quran is good and making sure that he is upon Tahaarah which means that he is clean and have proper wudhu on.
A child who knows the Tajweed rules, knows the proper rulings of salaah and tahaarah, then he may lead a congregation.
Allah knows best