I ran this for a single player and despite the large dungeon, we tied it up with a bow in 5 hours of play. It was a lot of fun and I highly recommend it.

The adventure features a twist ending that's easy to pull off if you just play Zareene as legitimately concerned. The dungeon is large and looks intimidating, but just keep the map open - the rooms are relatively self-contained, so just focus on where the PCs are - you don't need to keep the whole dungeon in your head at once.

One thing I'll note is that if Kobolds are a playable race in your world, you might want to adjust the focus of their villainy away from "there are kobolds here, oh no!" to "there are some kobold-supremacist cultists here, ew." When I ran this, I made the missing kid a half-gnome, and thus a prime target for extremist cultists, since kobolds have canon beef with the Gnomish god, Garl Glittergold.

This adventure is beautifully written, super easy to read, and I'd say can even be run at a glance, though it's best if you can read the NPC notes and skim the dungeon ahead of time - roleplaying the NPCs in accordance with the notes is critical for pulling off the twist ending.

Give Firestone Keep a try, it's a great start to an adventuring career! With Lord Blackmoor's good word, it's easy to hand them off to a friend of a friend of the Lord's for their next quest.