I came across this blog about traveling with disabled children. It has some great information. You can access it here http://www.specialneedstravelmom.com/.
Make sure you look her up and tell her where you found the information. Us parents of special needs kids can use all the resources we can.
Here is my comment in the passport section of her blog
I just came across your blog and even though this question is a few years old I thought I would give a reply that may help someone in the future.
I have 4 special needs daughters. Three of these are adopted. There ages are now 27-44. I am 69. Three of these girls are Down Syndrome and one has Fetal Alcohol Syndrome. They have different ranges of abilities.
We have traveled all over the world and our latest trip was to the Mediterranean, on a cruise, with 2 days in Venice and 2 days in London.
The girls have had passports for years but when I went to renew my youngest one’s passport she was going from a child's to an adult's. They didn’t know what to do because she couldn’t really sign her name. She makes an attempt but they were not sure if it would pass judgement. So they told me to sign under. Then they told me no, she can do it herself, then they ended with me signing as her mother, under her name. The passport was denied, even though she had one before, but as a child. It was denied because I am only a mother, not a conserved or legal guardian. This is because she is an adult. So I wrote my local Congressman, who had given us a private tour of the Capitol a few years before. They immediately got in touch with the Passport Authorities and within 24 hours I had her passport in the mail. The Passport office called personally and asked if they could help me in any other way. So sometimes you just have to keep working at it.
Also I have one daughter who can't frown. Her previous picture had a smile but this time around they didn’t like that. So we had to try and make her frown. Her passport now looks like America’s Most Wanted, but at least it gets her to where we want to go.
This is what she really looks like.