This is still the problem, then. The damage is based on the amount.
You could demonstrate that kidney cells die when exposed to cyanide, and apple pips have cyanide in them, and you might say "kids are exposed to kidney damaging cyanide through this mechanism. The extent of the damage remains to be seen".
If someone challenged that by saying that kids who eat apples aren't all dying of cyanide-induced kidney failure, you could also say we're just not seeing easily detectable ones.
But you'd be absorbing (and re-emitting) entirely the wrong impression without the supporting contextual info that genuinely links the two together. We don't create more and more unusual hypotheticals to explain a belief in things or effects that we don't know exist. We demonstrate why we think they should exist and go from there.
We're not seeing them having severe kidney problems - easily detectable ones. They could still have damage or accululated crap on their kidneys.
In any case, the article doesn't say kids kidneys die in 2 days. It says lab kindey cells die when exposed to such microplastics, and microwave setups produce those microplastics.
Meaning: kids are exposed to kindney damaging microplastics through this mechanism. The extend of the damage, as the article also states, remains to be evaluated.