Which of these two statements is correct?
Did you know 1 in 4 kids have an undetected vision problem?
Did you know 1 in 4 kids has an undetected vision problem?
Which of these two statements is correct?
Did you know 1 in 4 kids have an undetected vision problem?
Did you know 1 in 4 kids has an undetected vision problem?
You want to use "has," as the subject is one kid, not four kids. This is the kind of thing where the technically correct version doesn't sound as good to our ears as the alternative, but the logic makes sense if you restructure the sentence: "Did you know that in 4 kids, 1 has an undetected vision problem?"
Since your sentences refers only to one kid, the verb should be "has."
"Have". Fractions are grammatically plural -- 0.25 kids on average have a problem.