Question:

What is the ruling with small non-baaligh kids standing in the first saffs next to their fathers and other adults? 

Answer: 

The first thing to bear in mind is that very small children (i.e. below the age of seven) from whom one fears a racket or a mess in the Masjid should not be brought to the Masjid.i 

The primary ruling, when it comes to children, is that they should stand in a separate Saff behind the Sufoof of the adults.ii