while some are echoing Mom and Dad at home and complaining about this "new" math.

It isn't new, though, is it? Solid, traceable reasoning has always been the basis of mathematics. The fact that a man once spent more than two hundred pages just to prove, with fundamental footwork that even religious fanatics shall envy, that 1 + 1 = 2 seems to prove it. It's the lack of mental rigor that undermines the solid structure that mathematics is based upon, which is what your kid's Mom and Dad have aplenty of.

Tell them "Your lack of mental capacity to understand the necessity of hard mental work when it comes to hard science shouldn't undermine your child's future capabilities" next time someone complains about "not being a maths person". I'd love to see their faces when they hear that.

You're doing divine work. Bless you, and best of luck with your work. Don't give up because of a few bad apples: dust accumulates all by itself, but people who make effort to shine are what's worth looking at exactly because they don't stand to stay filthy.