Walking in front of a Musalli

It is not allowed to pass in front of a person who is performing Salah even if one has to wait for a considerable amount of time. Waiting would be better than passing in front of him. According to the Hadith, if a person knew the gravity of passing in front of a Musalli, he would have preferred to wait for 40 or 100 years...

The importance of filling the Saff

... But if there is place in the Saff which is not yet filled, then one must fill the Saff, even if one has to pass in front of a Musalli to do so. Through this, one sees that the gravity of leaving an empty place in the Saff is more than the gravity of passing in front of a Musalli. That's why it is authorised to remedy what is more serious by committing what is less serious...

Children in the Saff of elders

...That's why it is not good to allow minor children to stand in the Saff of elders because it is as if there is no one in the place where the minor is standing and thus that Saff will be considered incomplete. In a way, allowing young children to stand in the Saff of elders is more serious than passing in front of a Musalli and it is likened to leaving an empty place in the Saff which Nabi (Sallallaahu Alaihi Wasallam) did not approve at all. In this sense, allowing children to stand in the Saff of elders will be more serious than passing in front of a person who is performing Salah.

[English version of "Traverse divant ene mousswali" – Al Jamiat – Vol.5 No.12]