The thing is, there's already way too many wizard kids for the White Council to handle. They're not going to have some conceive-a-thon to make more when half the ones that are already out there end up at the wrong end of a Warden's sword because the Council couldn't get to them on time.
In White Night, the idea wasn't that killing 20-ish practitioners would cripple the magical world. It was that the actions in Chicago were a proof of concept that they could get away with it on a world-wide scale.
Magic being passed on through the mother is a tendency, not a hard and fast rule. Nowhere is it stated that a wizard mother will always have a wizard kid, nor is it stated that a wizard can only come from a magically talented mother.
Harry's grandmother was not a wizard, for instance.
Sometimes it just happens out of nowhere.
Thomas can, in fact use magic -- he works up a tracking spell in "Backup" -- he just doesn't have the same kind of talent that Harry does.
This has been discussed before. Word of Jim has suggested that Molly has a talent while Daniel does not because Charity still had some of her magic while she was pregnant with the former, while it had faded almost entirely when she was carrying the latter.
The closest we've ever come to a real conclusion is the idea that it's not so much whether the mother is a wizard as it's whether she has exposure to magic while pregnant.
And, again, sometime it just happens.
OP: catch up with the series. There is information you do not have, and TV Tropes does not adequately cover all events.
Regardless, I don't see the plot hole. Are you saying that Harry's line about magic usually passing down the maternal line (and that it's apparently well-known) not prompting the White Council to promote a conceive-athon is a plot hole? Because there are tons of other lines of dialogue (including quite a bit in White Night, if I remember, or maybe that's Small Favor/Turn Coat) discussing that the White Council does not have the manpower to manage the wizard-level talents that are springing up as it is. Harry specifically mentions that the wizard population is undergoing a massive expansion (though it's still a fraction of the total population, as most subgroups are) at the moment.
Wizards are also extremely long-lived. They have more time to have children than vanilla mortals do. There is less of a need to rush. Couple that with, as Mr. Death pointed out, big talents can spring up from magical have-nots (the typical patrons of Mac's, for example), or even out of nowhere, and you try to manage that with a few hundred people. There are way, way, way more sensitives in the Dresden Files than there are wizards. Not only are there too many for monitoring all of them to be practical, there had never been an effort (or means, for that matter) to identify all of them. That might change with the Paranet, but I doubt even that organization will be able to identify those like...eh, you haven't read Changes yet. But there is a side character with a small talent who would never be noticed by the Council, and would be equally unlikely to have ever even heard of the Paranet (at least... well, read Changes.)
Anyway, it's not a plot hole however you look at it, unless I'm completely missing your point.