Not the parent but I had similar experience: I could open an account by ING.

Opened end of 2022, fully online. I have italian citizenship; my residence title was just... resident? I had the Anmeldung for the apartment I was living in.

But it wasn't all easy. I needed:

- A german telephone number.

- A document (with a MRZ) with my german address in it.

- Not sure but maybe they also asked about my job?

The document was critical for me; I had to go to my embassy and apply for a new id that had my german address.

Also everything was fully in german, no english whatsoever.