It is not prohibited for believers to be outside at the time of Maghrib. Indeed, it is required for men to go out at that time, since they have to go to the mosques to perform their Maghrib prayers and then go outside again after prayer in order to return to their homes.
We are encouraged, however, to keep our children indoors at this time, for the Prophet (peace be upon him) said: “When night falls, keep your children indoors, for the devils are out and about at this time. When an hour of the night goes by, you can let them go.” [Sahîh al-Bukhârî: (3280, 5623) and Sahîh Muslim (2012)]
The Prophet (peace be upon him) also said: “Cover your dishes, close your containers, shut your doors, and keep your children indoors at nightfall, for the Jinn are out and about and stalking.” [Sahîh al-Bukhârî (3316)]
The meaning of “hour” in the above hadîth is explained by the following hadîth where the Prophet (peace be upon him) says: “Do not send out your cattle or your children when the sun sets until the blackness of the night descends, for indeed the Satans are sent forth when the Sun sets until the blackness of the night descends.” [Sahîh Muslim (2013)]