I haven't to date, but why not? If I'm carrying a child they're well above my waist and well away from my gun, their hands are on or around my shoulders. Sure I'll be conscious of it just to be on the safe side, but the way current smart-gun tech works (RFID) if they grab my gun why I'm holding them they'll be holding it close enough for the RFID to register; that's already assuming the fantastic case of a child I'm holding turning upside-down, reaching deep into my pocket and pulling out an 18 oz gun without my noticing. Pretty sure I'd have to be drunk for that to happen, and if I'm drinking heavily my gun is locked up well beforehand.
I agree the threats are moronic, and in a perfect world yes such technology should be available. The fear is of those who would exploit such technology to enact a defacto ban. The Armatix costs $1400 by itself, with another $400 for the RFID watch. There are a few state legislatures who would absolutely love for every new gun sold to cost $1800, while covering up their real intentions in the name of "common sense safety features". By contrast the most popular guns on the market (Glock, M&P, XD, etc) are less than $600. California's already trying something similar, albeit not as extreme by requiring micro-stamping; a measure currently being challenged in court.
Sure such measures probably wouldn't stand up to court scrutiny, what with SCOTUS ruling that the 2nd amendment protects guns "in common use", but that's a road the pro-gun community doesn't want to go down; just another political battle of attrition; and per the New Jersey law alone it would be forced to go down if such guns became available.