Oh MissBetseyTrotwood 
You don't have to attend the appeal hearing, but it greatly helps you if you do, because you get to say your case, which can give emphasis to what you feel are the pertinent points.
If you attend the appeal hearing the panel will ask you questions. They should be kind and sensitive though. Their job is to help you give all the information you can, without assuming what that information will be.
In terms of this appeal, it doesn't matter about letters saying he's too young for his year - the panel can only consider if he should have a place in Year R, his year group.
The chances of success, unless a mistake has been made are very, very, very, slim. I can't say 0, because you never know what will be unearthed on the day. However, an appeal for a Year R place in a class that has a maximum legal size of 30, which is already at 33, with no mistakes on the part of the LA, is going to be almost impossible to succeed.