Program Evaluation
It can be difficult to assess the evaluation of in general of service to a specific population, but there are a varities of ways in which both quantitative and qualitative evaluation can be gathered.
Statistics
By "measuring library resources" for visually impaired children and young adults before changes and then again after making changes, even if no new purchases were made, will help account for your goal of better serving those with visual impairments (Dresang, 2006). Dresang then makes the statement that outputs of how these services and resources are used should be made. This can be done by keep track of circulatlion of works that have been classified as resources for "visually impaired." Another positive use of these statics will allow you to keep track of all peoples use of these materials, not just those that are blind. While the intended use of e-books and large print maybe for visually impaired patrons, a greater access to information can be gained for others, and can be reflected in these statistics.
Surveys
For particular programs, you maybe able to provide a survey at the end of it to see how patrons felt about it. A simple survey of a short number of questions is a great way to quickly document results without overwhelming patrons. Especially for programs targeted for visually impaired children, it will be key to see there perspectives and then adjust to their needs.