I found the test a bit longer than 5 minutes, but I had to read the directions first...if you take this test with your kids, read the directions then call them over!
I also found the skills to test a bit higher than my expectations, but I understand they are based on No Child Left Behind standards (we'll leave the commentary on that to another day)
On this page is lots of 'why you need us' and a link to a test that will ask for a name and approximate grade level (though I do not know why since you then have to take a pretest before it will send you to the actual grade test...)
Might be a direct link to the test.
reading test
The first 9 weeks of each grade are free to download and print, as well as some of the extras.
printable links
Scroll down for each grade (including kindergarten level) look for clues like 'worksheets' to find the free part.