So, what you are saying is that the child would only need to be registered at the UK consulate. This is good. The idea of being British, American and Brazilian, with a Russian wife and an Italian child doesn't really appeal to me! (and this registration is also conditional). At least let two of us have the same citizenship!
As it is I might need to naturalise myself Italian, so that my wife can cease being a second-class citizen, but I can only do that in 4 years' time and then she'll need to apply herself as the wife of an Italian.
Now, imagine the hassle if we were then to move to, say, Denmark, next year!
You remember me, JAJ!
Yes, they can be American, but only naturalised (and not straight registration) through my mother, as long as I can prove she lived in the US for a period, which I might be able to do, if I manage to get hold of any of her old documents.
The Brazilians allow for a "provisional" registration, which must be confirmed before the age of 18 by registering again in Brazil. Since we shall not be going there (we would need to register our marriage too, which involves my wife needing to apply for documents in Russia) if we can possibly help it.
And quite frankly, what interest is it to have Brazilian or Russian citizenship in the EU?
I imagine that if these children were born in the UK that would solve the problem, wouldn't it?