I would say bring the stool, and if anyone seriously has an issue with it, you can ask them to kindly go re-examine why they're volunteering for FIRST. There's a difference between enforcing the letter of the law on an authority trip and letting the little things slide.

Give the kid the experience of being on the competition field. It's a one of a kind experience, and not one that should be easily discarded for some rule about bringing a stool in.