The general rule is that only dogs that are entered for show or registered as not for competition can go. Some shows provide facilities for non entered dogs to be left in RSPCA facilities at the gate - they wouldn't take such a young pup.
At less formal open shows you can often get in with a dog not entered - but would not be wise to take such a young pup - imho. Champ shows are often much more formal with dogs being checked in and out again.
tbh - unless you are showing - its usually best for dogs to be left at home anyway, especially if it is a big show and you want to stay a long time. Big shows can get very very busy, and hot!!. Again - just my opinion.
hth