Finnish acquisition check-in: 8-year-old; 13 months in Finland
When I was a kid, two of my favorite books were Searching for Shona (which I have never come across again) and Calico Captive. In both of these books, a young child moves to a new environment (rural England due to WWII and Abenaki lands due to kidnapping in the 18th century, respectively) and forgets his/her previous self completely. In the first book, Shona doesn't remember who she was before the war, and in the second book, Sylvanus eventually forgets how to even speak English.
These book characters (Sylvanus is actually real, though) remind me of Magdalena. She has taken up Finnish and never looked back. Her accent is near-native if not altogether native. She can say what she needs to. She says she thinks in Finnish and dreams in Finnish. Sometimes when she opens her mouth, Finnish comes out first. There have been a few times where she did not know how to say something in English: one time she told me she "successed" (succeeded), and another time she said that when it was cold, she got "chicken pops" (goosebumps).
A few weeks ago, the kids and I were at a day camp activity for orienteering. It involved a mid-day lunch served in a cabin. I was helping Miriam and Sterling get themselves served and realized I didn't know where Magdalena was. Then I saw her: she had already served herself and was eating at a table with a bunch of other kids, chatting away. She was completely, linguistically at home in that environment.
All that said, her grammar is still a little rough around the edges. But I'm sure that will come with more time.
It seems the challenge with Magdalena may be keeping up her skills in English, especially academic English. That's why I'm so glad she's at a bilingual school that will eventually support her in that area.
Pro tip: if you want to learn a new foreign language, do it when you're 7-8 years old.