What is meant by "restrain your children" in the Hadith: "When night falls, restrain your children, for the devils spread out at that time"; does it mean preventing them from going out after Maghrib or going to play at the club?

"The first part of the night" is meant by "when night falls," and during this time, children are commanded to stay indoors because devils spread out then. The wisdom behind this is that children are often accompanied by impurity and lack devotional recitations (adhkar), and devils attach themselves to whatever they can. If the hour of 'Isha' has passed, which is the first hour of the night, then there is no harm in letting them go out.