...But I still wonder if this kind of technology is effective enough for those minors who use their parent's access accounts to gambling sites. Will this be able to detect such action?
You have to be in the tech industry or at least try their service if you really want to know the answer. I'm giving the company the benefit of the doubt in determining the authenticity of documents provided through the KYC process and cross-checking with selfies. It's probably over the top for Germany to hire a third party to do all these but it's all good if they are efficient and effective.
Will this kind of technology can trully protect minors and prevent them to access gambling sites?
Just site access? Not completely. Maybe yes if we're talking about depositing and playing.