You see, in that sentence, you're singling out the fat kid, right?
That's the "one in" part of the sentence, you're focusing on that child, which makes it the subject. Therefore because it's "one kid", it's singular. Same for "one percent", they're both still one of something.

If it were "four out of ten kids", it would be a plural.

The mistake occurs because people see the "out of four" bit and think they're talking about a group, when they're not.
( Kroney, Thu 18 Apr 2013, 10:54, 1 reply, 12 years ago)

"you're singling out the fat kid"
you're not, you're describing a proportion of kids as fat.

"The mistake occurs because people see the "out of four" bit and think they're talking about a group, when they're not."
They are.
(PsychoChomp, Thu 18 Apr 2013, 10:57, Reply)

And this, Chompy, is why you're looking like a fool.

( Kroney, Thu 18 Apr 2013, 10:58, Reply)