This may be all one fic or I may be mixing up two. Angel takes Xander in as a teenager because his home life is unsafe. Angel sets him up in a basement apartment next to his and acts paternal. Later does same for Faith after discovering her living in an unsecured motel room. At one point he lectures on human nutrition and makes Faith eat broccoli. Possibly in same or different fic, Cordelia is taken in as well. Possibly in same or different fic, Xander and Faith go into a warehouse (looking to scavenge weapons, I think) and are distressed to find demon snack food made out of kittens.