If you check out the disABILITIES Board, post #6 is all about Guest Assistance Cards (which is the name for what you are asking about). It is not a pass (meaning it doesn't give extra of faster access ). it is a card that tells the the Cast Members (CMs) that you need some extra assistance for your needs. You may get a card or you may get a sticker to put in the stroller that identified it as a stroller being used as a wheelchair.
You do NOT need a letter from a doctor. They are not allowed to ask for one as proof that you we assistance related to your disability; this is part of the ADA (Americans With Disabilities Act), so anyone who tells you that you must have one is not aware of the law (which passed in 1991)
You may choose to bring one if it makes you feel more comfortable, but that is up to you and the CMs might not look at it at all. In fact, most people who have posted about taking one said that the CM did not want to see it. If you bring a letter, it should focus on your child's needs not on the diagnosis that is behind those needs.whether you bring a letter or not, you need to be prepared to talk about your child's needs at Guest Relation. There is more information about this in the disABILITIES FAQs thread in post 6. The thread is near the top of this board or you can use the link in my signature.