So, the ten-year-old's father tells me that our last addition will be playing a Monk. (In Dungeons and Dragons terms, that's basically a martial artist: unarmed or lightly-armed combat, no armor, no shields, not able to soak up damage the way a Fighter or Barbarian can but more mobile and versatile and perfectly capable of dishing it out.) This is not a bad addition. The group could really use a tank, but if they can get some patterns and strategies together I think they can make this work. They just need to remember that they're not a toe-to-toe melee group; they need to rely on mobility, flexibility, stealth, and magic.
It's going to be interesting, because this dungeon is heavy on... well, no actually I shouldn't say; Firstborn does sometimes read this blog.
I'm still thinking about throwing an NPC fighter in with them, but I haven't decided on that yet. I may see how they do the first time they venture into the real dungeon before I make up my mind. It can be awfully fun to watch a non-standard batch of adventurers figure out how to make things work, and I'd like to see them sort out the party organization on their own.