When we went through security last time I told them she would set off the alarm. She was connected to an IV pump, but I didn't tell the first people that, they didn't care why she would set off the alarm. They has her go through a seaparate panel to get to the secure section, waited for me while I went through the normal metal detector and then went over to her, and then they did the hand exam. I had my DH take my DS and the carry on's other than my purse so I only had to focus on DD which helped.
I would hope if you explain the situation (she will set off alarm which will scare her because of Down's), they will not make her go through the main one and will be willing to just do a hand inspection. You are asking for a "medical reason" and not just because you want to. However I have certainly traveled enough to know things often aren't done the easy way, or the smart way for that matter.
Good luck and have a wonderful trip,
Suzanne