Will NPC's start a relationship?

@catandlenny, when you say "vampire child" do you mean a vampire teen or adult child of one of your Sims, or do you mean a vampire elementary school-aged child of one of your Sims.  No Sim of the Child life stage can flirt or be flirted with 🤒 .  As for teens and older Sims, they can be flirted with, of course, which you probably already know! LOL 🤭

As for the NPC's, it depends.  If they are like the Mail Carrier, The Repairman, The Maid, etc. they won't flirt while they are working.  You can invite them to hang around and then they could flirt with you if they have the flirty character trait or you watch the Romance Channel on TV together, but there are no guarantees.  Sometimes Service NPCs will get stuck at your house though.  They might show up at a club, restaurant, or festival or park around town, but this is more hit and miss, unless you invite them yourself.  But you can always invite them somewhere as friends and then see where things end up.🤨

As for other NPCs that aren't providing services (Townies), they can show up at these places too.  But even if you engage in conversation with them, flirting isn't likely to happen, until you know them better or unless they have the flirty trait.  If you have the City Living Pack, then a great idea is to go to the Romance Festival when it is in town and hang out with Sims drinking the Sakura Tea.❤️  To prevent your Sim from initiating the flirting, don't have your Sim drink the tea.  The tea puts Sims in a flirty mood, so any Sim drinking the tea is much more likely to flirt with your Sim!  

I don't know of any cheats to make another Sim flirt with yours, but there is a cheat to make them married, but you probably don't want to go about it this way for story telling purposes. You can always have them move in as a roommate/friend and then let things slowly progress, but then they wouldn't be a NPC Sim anymore. Also, If your Sims paints and is in a flirty mood on their own, have them paint a flirty picture and hang it in your Sim's home.  Turn the aura on and it will emit a flirty vibe.  Then invite the NPC Sim over and talk to them while near the painting and they will be more likely to flirt with you (unless they have the unflirty trait).  

I hope this helps you!  Please feel free to update us on what happens with your "socially awkward vampire child"!  Best of luck and Happy Simming! 🥳