it's not fair that everything i do comes out bitchy

a woman comes up to me at the desk asking if we have jobs or volunteer positions available for teenagers.
"yes we do!" i say enthusiastically.
then she tells me her son is autistic. so now, i am picturing someone with high-functioning autism. someone like corky from life goes on. someone who could easily put away books in the children's room of the library and be happy as a clam. but then she tells me that she and her son live right around the block, which is perfect because they are still doing "travel training" and he is not yet up to travelling anywhere. well, this changes everything. i'm sorry, but i feel that if riding the subway is too stressful for you, you cannot work at this branch. so i tell her that this branch is "sort of rough" and that it might not be the best place for her son. she looks at my blankly as if i am speaking utter nonsense.
"what are you saying?" she asks.
am i not speaking clearly? "i mean," i elaborate "that some of the kids are kind of rough here? and if your son is sensitive it may not be the best place for him?"
again, blank stare. "i don't know what you mean." now she is starting to get defensive. "i just want him to work somewhere quiet and peaceful. he can't really handle loud noise."
ah, now she has said all i need. "this place is not quiet." i tell her about screaming babies and young teens acting like houligans, running amok in the children's room. then she asks why can't he just volunteer in the adult area then. surely, that is a quiet place.
"ma'am, this branch is not the quiet serene safehaven you are imaginging it to be. here in the adult area would be worse. it's very loud here and stressful. i don't know if your son can handle it." i choose not to mention people screaming at the top of their lungs and locking themselves in the bathroom, fights at the circulation desk over fines, crazies muttering to themselves, junkies passed out in the foyer, etc etc. i just reiterate that this is not the best place for her son. and in my heart i am looking out for him. i am picturing a nice young man with autism who can't stand crowds and loud noises and is sensitive and will immediately freak out if he spent more than five minutes in here. i want to protect him. i wish someone would protect me.
but this mom just doesn't get it and then she leaves in a huff as if i am being unreasonable, when in reality i am just saving her from heartache.