ROTGxAOT Attack On Nightmares

I had an idea, so if you don't mind, here it is!

Jamie (Eren, later armin), Sophie (Mikasa), and Jack (Arimin, later Eren) are three kids living a pretty normal life in the walls of Burgess. But when the pitch black Nightmares infest the town after 100 years of inactivity, it's time for something to happen. With the death of his mother, Jamie swears revenge, however greatly leans on Jack for help. At this point, Jack starts to become the main character when he helps Jamie and Sophie into the joining the fight to defeat the Nightmares. They meet the other kids (who will be Sasha, Connie, etc.) and so on so forth. Jack's hair is brown as he grows up, but oneday he gains the ability to turn into a Frost Nightmare, and only when he returns to human form, his hair turns white.
Also, the guardians will be the people like Levi and Hanji.