This society values people by their 'contribution' to 'society,' meaning their ability to work or provide social cohesion. Individuals who do not easily conform to having roles like these due to 'disabilities' are often though of as 'free loaders,' but with a type of pity that allows social welfare/charity/tolerance/etc. to be given to them.
You're making the assumption that these people have nothing to 'contribute,' that their perspectives, attitudes, and overall character would not be something others could appreciate or find value in. (ugh, fuck the english language, I hate using words like 'value' in this way, but you know what I mean.)
Kelly Rose Pflug-Back wrote a very interesting piece addressing this same issue in an issue of Fifth Estate that came out a few years ago. While it primarily dealt with individuals with 'physical' disabilities, I believe it applies to this question in addressing the perceived 'lack of value' that society frames the way we look at people with these types of conditions: http://www.fifthestate.org/archive/387-summer-2012/survival-fittest/
About the actual practice of supporting people who may need it: I wouldn't see an 'anarchist' society as being one with closed-off nuclear families that privately deal with situations like this. The reason humans group themselves in this institution today is primarily due to the demands of the economy, where a stable family unit provides shelter for workers as well as begins the process of discipline for children to prepare them for the capitalist world. I don't want to make assumptions about what an anarchist society would look like, if we could even get to that point, but I'd guess that a healthy way to raise a child with we now call 'autism' would involve other family, friends, and people that, for their own interest and the interest of those they care about, help each other out. (dare I say it?!...COMMUNITY) It wouldn't have to be a private affair. Nor would it be a burden: I would also guess that the subjectivity that the autistic person experiences the world through would probably be of value and interest to said people as well.