I dunno, that's a very open-ended question. it obviously depends on the age of the kid. In your example, probably a five year old or older could understand and even formulate that idea. but I don't think most children can get past 1000 level (at best) before age 8 or 9. or course prodigies are the exception, but we're talking about ordinary kids.
Do ordinary kids have the ability for rational thinking?
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Children are cleverer than we think, as this book explains.
"Attack a weakness" is probably intuitively understood even before someone can talk.
The difficulty in chess is sometimes it's hard to tell what is or isn't a weakness, and sometimes it's difficult to know how to best attack a weakness.
With a lot of experience you can, for example, ignore counterplay that doesn't work, often without noticing the non-working counterplay even exists. It's just invisible. The lower rated or less experienced players struggle to filter out these sorts of things, so ideas that obviously work to you can be difficult for them to judge.
Like for example, " I see there is a weak pawn, I will attack that weak pawn with my pieces."