Hmm... has the MG story been published? That may make a big difference.
Seems risky to write a darker, "grown-up" tie-in to an existing MG work. I've seen younger-aimed spinoffs to existing universes (Gail Carriger wrote an MG/YA series tying into her grown-up Parasol Protectorate series, off the top of my head), and I've seen MG series that "aged up" to YA over their run, so you can mix age categories in the same franchise... but that may be something you can only really get away with if you have sales like, say, Gail Carriger. For the rest of us, it might lead to brand confusion, especially if there's a big gap in tone; Carriger's universe is a bit tongue in cheek and light, at least what I've seen of it, so it's not like she was going from GoT-level grimdark to MG (which can get twisted and dark, but generally not that twisted and dark.)
If the MG story hasn't yet been published, maybe re-examine it to see if it's really MG or if it, too, wants to be darker and "older." (You can have MG-aged main characters in a book that otherwise is not aimed at MG audiences.)