No, Arinsu's statement was "He's still this strong even though he's become a child," (gaki ni nattemo soko made tsuyoi to wa ne) not that "he hasn't lost power as a child." You're twisting words at this point. Not to mention Goku didn't digress when Shin said he wasn't at his prime. Both statements are a far cry from, say, the Perfect File statement that Goku hasn't lost power when he became a child in GT. You'll notice even while Shin states that Goku acclimated to the heavier air in the Makai already, he never states he's just as strong as he was before being downsized.