In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
In responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
In the case of a male praying behind another, the follower should stand one step below the imam on the right side.
If another comes and joins later, they should go back to form a line behind the imam.
If the wife is praying behind the husband or a daughter or sister praying behind her father or brother, she should stand a few steps behind the imam.
The Prophet (peace be upon him) was praying Tahajjud one night. His cousin Ibn Abbas, who was a teenage boy at the time, woke up and joined him while standing behind him. The Prophet gestured him to stand right beside him on the right.
Having said this I must state emphatically that the above rule must be qualified because of the unprecedented situation of COVID-19.
In this situation, we ought to observe the strict recommendations of the health officials with regard to forming lines in Prayer as well as other necessary preventive measures.
Therefore, lining up closely behind the imam on the right side should be done only if the imam and the follower are members of the same household. In that case, the standard rule should be followed.
If, however, we are speaking of praying outside our home, even if it involves our own children or relatives who are not living in the same house, then we must keep the rule of two meters distance while lining up.
So, even if it is one person is praying behind another person, the follower must not stand closely behind him, rather he should stand two meters away behind him on the right side.
When the normal situation returns, the original rule mentioned above should be followed.
Almighty Allah knows best.