Yes in a 200m foot race someone who is 5-10% faster is very noticeable. But by that time Usain Bolt was already a well trained athlete competing in organized events, so of course he was noticed.
My agreement can be broken down into 2 arguments.
1. If we accept that Usain Bolt's genetic speed boost is the same throughout his life, so that at 4 years old he had a 10-15% boost compared to other other fast 4-year-olds with similar fitness levels--I'm saying that given the kinds of running and games 4-year-olds play, that isn't a big enough difference to guarantee that someone notices it. That is, it isn't guaranteed to swamp out other variables. Bolt at 4 wasn't likely faster than his 6-year-old cousin. Hypothetical fat Usain Bolt at 4 wasn't likely faster than his fast 4 year old friend. Hypothetical indoor sheltered Bolt who never played outside, probably wasn't faster than his fast friend who ran outside all day long.
2. We shouldn't accept that his genetic speed boost stays the same throughout his life. It's very unlikely that Usain Bolt was the fastest 9 month old in the world. It's very likely that at least some of the physiological differences that enable his freakish speed didn't develop until puberty.
Kids generally stop running around for fun outside of organized activities well before then. Potentially giving hypothetical Usain 2, plenty of time to develop other interests.
A deconditioned couch potato 12-year-old isn't going to be lapping his peers who play soccer every day after school, no matter his genetic potential--especially if he doesn't care enough to really try.
The current 200th highest world record for a U18 is 21.04. Usain's record is 20.13 (my mistake). In the 200m you're generally looking at around 4.2 strides per second with each stride covering around 2.4 meters. So we have all we need. The difference at the end would be (21.04 - 20.13) * 4.2 * 2.4 = 9 meters, or more than 30 feet - multiple car lengths!
And that 200th fastest 17 year old of all time is not just a "fast kid". He's a physical outlier several sigmas away from the mean that would be creating similarly lopsided finishes at nearly nearly every local, state, and even national competition he entered. But Usain is just on an entirely different level than even that. So Usain racing against anybody even remotely normal would look like "fast kid" vs "extremely slow kid."
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Basically it's not really possible to fail to notice this sort of achievement. In PE he'd casually run laps around everybody without even trying. And by football, I don't mean any sort of team event - just a bunch of kids grabbing a football, imagining some nets, and playing. Nobody would be even close to being able to keep up to him. Literally anytime there is any sprint movement in anything, he would be on top - by far.
As for the hypotheticals beyond this, I don't know. We might have to just agree to disagree. I tend to think people are drawn to the things they're good at. And with running it's basically impossible to not notice that you're this many sigmas away from the mean. It's not like we're talking the best Polo player.