When Can an Immature Child stand in the Row of Adults?

Q: Salaah-74: A little child performs his salaahs, but at the time of Isha, no other boys attend the Masjid (because of which he has no row to stand in) Can this boy stand in the row of the adults so that he does not have to stand all alone at the back?

Answer: When there are two or more boys, they should stand in their own row. It will be Makrooh for them to stand with the adults. However, if there is just one child of an understanding age, he should rather stand with the adults rather than stand all alone by himself. And Allaah knows best what is most correct. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2