What a sweet idea!
I have never thought of doing a set of birth supplies for the big brothers. My MW asked if she can give them specific things to do - like hold this towel, etc., at the birth. I loved the idea of them having tasks to help out!
I also have my own list of things they can do - like help get me water, hold the cup for me, greet people who are coming (birth attendants and photographer) and show them where we are, help people find things in the house, etc. Granted, my boys are now 8.5, 6, and 3, so a lot of that is really for the older two.
I also have a new toy/project for them that I'll give them when labor begins. It's a cardboard, child-sized castle that they can decorate. I'm figuring that (along with their other toys) will keep them occupied for the less-interesting parts of labor. I'm happy for them to come and go as they please, and my mom will be here with them to help out.
Trying to think of other things... What about a t-shirt that says Big Brother? New toys/crayons/coloring books/movie/books about a new baby/etc. are all good. Maybe a disposable camera (or any camera you don't mind him using!) to take pictures at the birth?
Post back on how it goes!