Your child now is minor, he is not even adolescent. Hence he should stand in fardh prayer in the row of children not in the row of men. However, if the children in the mosque create disturbance during salah then if your child stands somewhere at the edge of men’s row then there is no harm in this case. May Allah confer goodness and success on your child. Amen!
Allah knows Best!
Darul Uloom Deoband