7(Shadowhunters)

An Au where Magnus accidentally met a shadowhunter in training(around Max Lightwood's age) and saved them which lead to Magnus adopting and being adopted by all shadowhunters in training from New York.

Magnus saved the shadowhunter in training from a demon that appeared out of nowhere and made sure the kid was safe and guided them back to the institute or closer to a group of older shadowhunters but fearing that something like that could happen again(because Magnus may not like shadowhunters but he won't let a child die even if they are a shadowhunter), he tells her how to find him or give him a signal in case she needs help while making her promise to not tell the older shadowhunters about their encounter and what he just told her(he should have been more careful with his wording).

He didn't think doing this would led him to be called several times but for some reason Magnus found himself saving or helping many child shadowhunters quite often and when he finally realized what happened he had claimed those children as his own and give them protections that he only gave those he cared for.

No shadowhunter older than 16 knew about Magnus and how much the shadowhunters adored him until while during a patrol to teach the shadowhunters in training a few things, a big group of demons appeared and Alec got severely hurt(if it was only Alec with the shadowhunters in training or there were others is up to the writer) and the iratze wasn't working so they decided to call Mganus for help as they didn't know what else to do.

Magnus had been a bit mad when he noticed the kids and all the ichor but quickly noticed the hurt shadowhunter and Max clinging to him so ignoring everything else he put his hands to work and healed Alec.

Once Alec was healed, Magnus started looking every kid to make sure none of them were hurt before taking them and Alec back to the Institute.

How anything else would go is up to the writer but the kids would certainly fight anyone that tried to keep them away from Magnus.