Even if none of them spoke a word of English, they were all functionally illiterate, and all of them were aged 17+ it wouldn't matter as long as the first tier tribunal judge had factored it all in properly to her decision. (I appreciate she may have made a different decision in those circs).
On the face of it, this is a generous FTT decision, but not unduly so. The judge could have gone the other way, but having heard the evidence, gave proper reasons within the law for deciding in their favour.
Leaving the ECHR wouldn't make a jot of difference here. We would still need a route of family visa applications, with a proportionality assessment built in, if this were within the Immigration Rules. We would still have edge cases which raise the public eyebrow for being unusually generous (this one) or unusually harsh (see Amelia Gentleman in the Guardian on multiple occasions).
"Hard cases make bad law" comes to mind. If we allow the existence of edge cases to drive us out of an otherwise very positive international treaty then we will have done ourselves an enormous disservice both at home and on the international stage.