I've been a judge at quite a few 4-H shows in years past and I remember the people in charge telling me how important it is to encourage the kids entering exhibits. There were 2 types of shows, the ones where everyone gets ribbons and prizes and the ones where you have to decide a 1st place, 2nd place, etc. Which type are you judging?
At one fair when I was judging the baked goods, those in charge told me that no matter what they tasted like, I needed to pick winners that 'looked' like winners. Otherwise, people would complain that they thought another item looked better and they would get nasty comments.
You can't make everyone happy but often times it's pretty clear which items are 'really good' and which ones aren't that great. You'll do just fine. Have fun with it.... don't let it stress you out. You'll be able to figure out which is best if it's one of those 1st place, 2nd place kind of shows. After all, they picked you to judge so you ARE the one to do the job.