All excellent questions! I'll try to answer them to the best of my ability.
I may very well expand upon this setting in the future and allow for other stories, but for now this is the only one that I have given much thought to. It has been stuck in my craw, so to speak, for quite some time now.
Firstly, the general public is completely unaware of the Sweep's existence. A city-wide curfew is always in effect, but it isn't strictly regulated by law enforcement officials. The need to be inside and behind closed doors by nightfall has become so ingrained into the culture of Gateshead that very few even question it. The fairy tales told to children are simply a reminder, and a tool used to ensure that each successive generation continues to stay inside obediently once the sun has set. Those who do not generally wind up dead, as they have no idea what to expect. Now, I will note that Father Baich is not the one who will be training them to work against the Sweeps. Most adults are far too cowed to dare go out at night. There is a very good reason why the protagonists of this story must be orphans.
This is probably something I should have been more clear about. Combat is perhaps the wrong word to use in this situation. Survival is a more apt way of putting it. The Sweeps are on an inhuman level. The ultimate predators. In straight up combat--without the use of heavier ordinances, which I will come to later--the orphans do not stand a chance. That includes attacking one all at once. Emphasis is put on avoiding and escaping, rather than out and out brawls. That will come much later. I should not that the plot of the story will not be strictly about defeating each and every last Sweep. That is an undertaking beyond the scope of the orphans. The day will be the time for the orphans fists and combat skills to really shine, when they are performing jobs, searching for materials, or otherwise exploring the city. Once night falls, running is the best available option. Think horror, where the victims rarely actually fight against their pursuer until the ultimate climax.
Weapons are whatever happens to be lying around! Fists, pipes, hunks of rock or brick: basically anything you can find can be used in melee. Gateshead is rather strict when it comes to gun control, but eventually the orphans will need to obtain some...likely by way of sublegal activities. Heavy ordinances are also within the realm of possibility, though a Sweep would have to be otherwise distracted for it to be any use. Each one is a puzzle in itself, with different habits and traits that work both for and against it. It will be up to the orphans (and myself, with small hints) to figure these out and exploit them.
Though I want to keep things semi-realistic, I cannot ignore the fact that much of Gateshead falls into the fantasy genre. The level of technology is far higher than it should be, as is often the case in steam and clockpunk settings. Creations are only limited by imagination and availability of materials. By the end, it is fully within the realm of possibility for the orphans to have constructed some sort of mobile fortress, or a kind of 'mecha'. It is at that stage that physics would begin to get thrown out the proverbial window, and allow the orphans to fight on almost-equal footing with the Sweeps. In any event, sheer brute force will not be enough to combat all of them, and some will always remain impossible to defeat.
Unfortunately, I realize that I have a great deal of work still to go on the mechanics of this game. I have to figure out the best way to lay out the city, how to direct the plot past the beginning to where I'd like it to end, and how to go about making building materials genuinely difficult to find. It wouldn't be very entertaining if they managed to just find a scrapyard and get everything they could possibly need to build weaponry! These are still young kids we're talking about here, not quite adults. The mechanics involved here would probably be beyond some of them at the start. If you (or anyone!) happens to have some suggestions concerning this problem, do let me know! This will be the first game I've actually run, so it's a bit nerve-wracking.
That being said, the actual start may be further away than I had initially projected for myself. I do not have a set number of people at the moment, though I would say no more than six players. Each player could of course create multiple characters, if they feel confident enough to maintain them to a satisfactory degree, and everyone can create side npcs and the like. Anything someone isn't sure they should create can contact me directly for input. While I have set up the framework, I am fully open to others working with me to flesh out any nooks and crannies that I've left in place.