Eh, Penne breaks my sense of immersion a little.
It's vaguely possible for there to be a highly competent 10yo yakuza granddaughter.
And yeah, then it makes sense that she fixates on the also very young MC. That's allowable plot setup.
But having a 10yo mob enforcer strains credulity; you need, like, actual strength and reach for that. Even if she has a magical bloodline or something, an adult with that bloodline would still be five times better at the job.
And, yeah, it's possible that the reason their outstanding talents are old or very young is that the generation in the middle got absolutely decimated in a yakuza war, but you can actually recruit new talent, y'know.
From a character design perspective, too, it would have made more sense to make Penne at least 15 years old, so there's a stronger visual identity and she gets more of a 'big sister' vibe.
Basically, the addition of Penne is evidence that the author just wants to add as many lolis as possible to the cast, and is willing to compromise the plot to make that happen. Or that the artist ignores the author's instructions and draws anyone below 20 as a 10-year-old.
Either way, feels like the beginning of a trend.
EDIT: checked out the source material, and as far as I can tell Penne's age wasn't given in the first 15 chapters. She's someone's granddaughter, but that still leaves a lot of margin.
Also, there's more focus on the MC's crafting in the novel; he detects poison with a magic tool he made, for example.