I spent about 2-3 months at the Marriott Marble Arch last year. I'd rate Edgeware Rd as probably safer (if a bit more foreign) than Oxford St, which is at its southern end. The stretch of Edgeware (which the Marriott is about 2/3 of the way up) from Oxford St to Marylebone Rd is mostly middle eastern, with many middle eastern eateries, groceries, and hookah parlors. It's heavily trafficked, both foot and car, though not as heavily trafficked as Oxford St.
Safety-wise, there are few areas of London I'd honestly call safe for a 12 year-old tourist out by herself (or himself) and Marble Arch isn't one of those areas. However, I didn't find it to be any more or less dangerous than the numerous other areas I was in.
The Marriott Marble Arch is quite convenient to a number of transportation options. The Central line is at the bottom of Edgeware at Oxford St (about 1/3 of a mile), the Circle, District, Hammersmith & City, and Bakerloo lines are at the top of Edgeware near Marylebone (also about 1/3 of a mile), and there are numerous buses that stop almost directly in front of the hotel.