The Internet is a major source of health-related information for parents of sick children despite concerns surrounding quality. For neurodevelopmental disorders, the websites of advocacy groups are a largely unexamined source of information. We evaluated treatment information posted on nine highly-trafficked advocacy websites for autism, cerebral palsy, and fetal alcohol spectrum disorder. We found that the majority of claims about treatment safety and efficacy were unsubstantiated. Instead, a range of rhetorical strategies were used to imply scientific support. When peer-reviewed publications were cited, 20% were incorrect or irrelevant. We call for new partnerships between advocacy and experts in developmental disorders to ensure better accuracy and higher transparency about how treatment information is selected and evidenced on advocacy websites.