"The child speaks slowly."
Translation:Το παιδί μιλάει σιγά σιγά.
What about optionals as "μιλά" and "μιλάει"? Should they be added or is the former colloquial?
yes, I'd say "colloquial''. While it's fine for the spoken language in a course we'd want the more formal version. There's not enough space to include much colloquial, regional etc versions of a language.