If a child stands in the same Saff as a man
Answer:
In the name of Allah, the most Beneficent, the most Merciful.
The ruling of the sequence of the lines in congregation salah is that the men should form the first row or rows, then the boys, followed by the hermaphrodites. Thereafter the females form the last row. (Nurul Idah p.79)
With regards to your question if there are two, three or more children then it is preferable to make a separate line for them behind. However if a child reads next to a mature man in congregation then there is nothing wrong with it. The Salah of the mature man would not be considered disliked. Therefore there is no need to repeat the Salah. (Raddul Muhtar p.314 v.2)
I have not come across any narrations or jurisprudence text saying that children should stand on the left hand side during congregational prayers.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham.