Is the time of Maghrib when devils spread, making it necessary to bring children indoors?

Yes, a number of authentic hadiths have been narrated concerning this etiquette, among them what was narrated by Al-Bukhari and Muslim from Jabir ibn Abdullah (may Allah be pleased with him) that the Prophet (peace be upon him) said: "When the night falls, or when you enter upon the evening, restrain your children, for indeed the devils spread out at that time. Then, when a part of the night has passed, let them go. And close the doors, and mention the Name of Allah, for indeed Satan does not open a closed door. And tie up your water skins, and mention the Name of Allah. And cover your vessels, and mention the Name of Allah, even if you only place something across them. And extinguish your lamps."

Likewise, Muslim narrated from Jabir (may Allah be pleased with him) that he said: "Do not let your livestock and your children out when the sun sets until the darkness of the ‘Isha’ (night prayer time) departs, for indeed the devils spread out when the sun sets until the darkness of the ‘Isha’ departs."

The scholars mentioned that these commands are to protect children and homes from the devils that spread out at this time, and that mentioning the Name of Allah is a means of safety from their harm.

Most scholars have interpreted these commands as recommendations and guidance, rather than obligations.