The site scope should definitely include dialects wherever they are spoken. It will be difficult enough to keep the proposal/site going without excluding dialects. (I believe some previous proposal used "Dutch languages" (plural) as a title for this reason.)
It may be a good idea to add Afrikaans, but be aware of the follow-up question: what languages are allowed for asking (and answering questions)? When the scope is just Dutch, I would expect English and Dutch as accepted language (French SE, Spanish SE and German SE, among others, do the same). If you add Afrikaans to the scope, will Afrikaans also be allowed for asking and answering languages. (It would be hard to disallow it.) If yes, should moderators have a more than superficial knowledge of both Dutch and Afrikaans (in addition to English, which should be a given)?