It’s not an ad hominem. It’s a polite way to suggest that you don’t know what you are talking about when it comes to teaching concepts to kids. I think it’s worth your while to wonder why it is so obvious that you are not a teacher. What experience and insights regarding age appropriate explanations are you missing that make it so obvious that you don’t teach?
Correctness, of the sort you are implying are absolutely not appropriate at all levels. There’s a reason kids in second grade are told that you can’t subtract a larger number from a smaller one, for instance.
1. None of what you are saying removes the ad hominem aspect.
2. In grades 1-2 (iirc) we applied negative numbers when we talked about the number line (which we had on the wall.) Even preschoolers knew about negative temperatures on the thermometer.
I don't know whether you mean "teach" or "tech" here, but this seems to be an ad hominem argument.
Regardless, I would submit that clarity and correctness matter both in teaching and in tech.