everyone knows that the one soft spot in Marcone's armor is children
Except, no, not everyone knows this.
Most people don't know this. Harry himself doesn't know this until
White Night. Heck, not even Gard seems to be 100% certain--otherwise she wouldn't be so "intent" on seeing evidence of it. Do you think a Freeholding Lord and head of a criminal empire can go around conducting his business uncontested if he has a known sentimental weakness towards such a significant portion of his city's populace?
As far as most people are concerned, Marcone discourages
civilian casualties in his criminal dealings, and children just so happen to fall within that category. Even Harry, even after a Soulgaze, assumes this is merely because Marcone wants to make crime tidier and more efficient, to discourage the police from pursuing him too fervently. It isn't until
White Night that Harry realizes that the comatose girl is Amanda Beckitt, and that Marcone has made it a rule to protect civilians for ethical reasons rather than purely practical ones.
Even then, Marcone's official stance towards protecting children isn't stated outright until
Changes. Then it becomes the major theme of "Even Hand" a year later.
(Full disclosure, I only gave the books a quick check before typing this out. Please correct me if I'm at all mistaken.)