What I don't get is why she was trying to get visas for the adopted kids, but not the biological child.
Either all 3 kids are British or all 3 kids need visas... it shouldn't be one thing for 1 child and the other for the other 2 children.
Maybe the couple asked the wrong questions at the start of this, and got off on the wrong track from the very beginning, and rather than backing up and starting over, every time they talk to a new person, they tell them what the last step was, so nobody's ever uncovered the fact that
none of the children is UKC.
Maybe she's unaware that UKC'ship is not automatic for her because she's UKC-by-descent, rather than born in the UK.
So they incorrectly believe that the biological son was born a UKC/USC dual national, automatically.
And they must believe that because the other two are adopted, they needed to do something to notify the UK of them, to secure their citizenship, but they haven't bothered to do anything until now because it was a hassle they didn't need to deal with until faced with moving to the UK.
So then dad gets this job offer in the UK, and the family are all, "Great! Mom's UKC, biological kid's UKC, dad has Tier 2... now we just have to ask about the other two." So that's all they asked about.
And based on the questions asked, and the lack of knowledge of the person they asked them to, they got bad information.