You make a good case but it is based on the supposition that this would be a possible way for mass murder. I am not sure that hundreds flocks to Lemonade stands.
But the whole point of course it to make sure that the enforcement commissars use common sense and reasonable discretion. They didn’t do that here. The notion that Islamofascists would use a 10-year old kid for mass poisoning is the same reason the TSA gives for a comprehensive search of grandmas in wheelchairs.
Secondly, you're asking a lowly field enforcer (in NYC this often means a political patronage gig) to make perfect on-the-spot common-sense judgements based on nothing but instinct, which is just asking for FUBAR trouble, IMO.
Thirdly, anyone who assumes terrorists couldn't possibly cultivate romantic relationships with blonde-haired blue-eyed women with cute ten year old kids from previous relationships is being a bit naive, to say the least. Just ask Israeli Airport Security - they NEVER make assumptions of innocence based on age, ethnicity or appearance when ferretting out bomb carriers. I found that out firsthand, in fact. As a younger single european-american woman travelling to Israel about 8-9 years ago, I got stopped, searched & interrogated extensively both coming and going - not because they necessarily thought that I was a terrorist myself but because there was a chance I may have been targeted for 'romance' by an undercover terrorist back in Europe (from where I was travelling to the ME) and used as a mule to carry death & destruction for him unwittingly. Those guys never make assumptions of innocence about anybody for a good reason - they simply can't afford to.