You likely won't find any advice for something like that, because it's not something that you're supposed to do in Apocalypse World. The player characters are always a step above the NPCs by design, they're in a class of their own; this is why it only takes 2-4 Harm to put an NPC permanently out of action but a whopping 6 harm before a player gets the option to come back with +1Weird. Don't worry about inadvertently nerfing them, because they should be weaker anyway.
In the situation you describe I'd have the MC either give the characters to some of the players, as if they'd picked the "get a second character" advancement, or just scrap the sheets and have them be regular NPCs. If the players are struggling with something that's one of their weak spots, then they need to make a deal with an NPC or attempt it anyway and deal with the delicious, delicious consequences that are the meat of Apocalypse World games. The MC really shouldn't be running them as PCs, because that results in them making rolls against themselves, and the MC shouldn't be making any rolls at all.
On a side note, NPCs shouldn't really considered be "friendly" at any point, they should have their own goals which are "just not that complicated" and they work with the players for as long as their goals align. It's one of the factors that stops the PCs - as overpowered as they are - from just steamrollering their opposition, because everyone is their opposition; even the Chopper's gang or the Savvyhead's workshop crew.
Hope this was helpful, I don't like having to tell you that there's no way to do what you want in this system but give it a go the way it's meant to be played, I promise it'll be awesome.