Probably too late now, but maybe useful for someone else for next year. My youngest took the CAT6/TerraNova last June when he was 5.5yo. It's really not that big a deal - he did the complete battery plus, so that's the longest version available, and he did the 1st grade level, but they also have a level for K - the instructions say to break it up into 2-5 sessions (I think - at least that's what the 2nd grade instructions say, and I think 1st grade was pretty much the same), we spread it out over 3-4 days iirc, and then even within that they say to give the kid bathroom breaks, etc at the end of individual sections. Most of the questions at the 1st grade level were read aloud to the kid, so the kid only needs to bubble in the correct answer (in the test booklet - they don't have a separate answer sheet until 4th grade). And they gave ample time, so the test should be done faster than the times listed.
I told my kids (oldest took the 3rd grade version last year) that they weren't supposed to know every answer - that if they knew everything, the test was too easy, so that probably helped with any potential test anxiety. My youngest is actually looking forward to this year's testing (just got the test delivered today), and was already talking about wanting to do the 3rd grade test prep book (silly kid). To be clear, it's not like he got a perfect score on last year's test - he was average in some sections, well above average in others, and well below average in word analysis - overall, he scored above average (81st percentile), but it's not like he liked the test because he aced it - he didn't ace it. I think it was just "something new and different", which can be fun, especially if you don't know there are people who are stressed out about tests.