Catch those children.
One of the missions you can attempt upon starting up Sequence 3 in Assassin’s Creed Syndicate is ‘To Catch an Urchin’. This mission requires you to break into a factory and liberate all off the child workers under heavy surveillance from the enemy guards.
As you approach the warehouse, you’ll see your first guard standing right outside the entrance. Wait for him to move, then climb up the wall of the warehouse and make your way into the building through the doorway atop the roof. Kill the first guard you see inside.
Head down the stairs cautiously, and make sure to stay in your sneaking stance. There will be one guard patrolling the next floor, along with the foreman, who you have to kill in order to complete the mission. If you have throwing knives, you can easily take the both of them out with a couple of well-executed headshots. There is a group of children you have to free on this floor as well.
After clearing this floor, head back to the stairs and go down. The next floor has only two guards on patrol. If you’re feeling ballsy, you can try to sneak past them and just head to the ground floor where the last set of children waits. Or, y’know, just take them out with more throwing knives or smoke bombs.
There will be two guards left on the bottom floor, along with one group of children. An easy way to do this is to stay in cover and whistle to attract the attention of one of the guards. As he approaches you, perform a cover assassination to take him out. With only one guard remaining, it shouldn’t be too much of a problem to sneak up and assassinate him from behind. Once you’ve taken both guards out, free the children and you’ll have completed the mission.