Hi Moses!
No, as a child I never ever had brain fog and was never sluggish. Luckily, back then, my mind was extremely sharp and fast. Now, with POIS I do often get severe brain fog particularly for words and sentences. When I don't have symptoms I'm back to my usual quick-minded self. I would guess from this that POIS cannot be predicted from childhood based on brain fog in childhood...but maybe I am an exception?