What are the chances of a child with generalized dystonia affecting mainly the feet and legs developing other types of dystonia, like cervical dystonia?

Possibly: Generalized dystonia can progress to affect other areas. However, cervical dystonia appears to be specific and not necessarily caused by progression from the generalized form. A pediatric neurologist might be able to help with diagnosis and possible treatments.

Need more info: Generalized dystonia implies that all muscles of the body are at risk. Dystonia musculorum deformans is a horrible and progressive disease. Less severe forms occur that are just intermittent. Often they are worse during growth spurts, especially at puberty. Good muscle management is necessary. Some medications help, and there are now very aggressive approaches including deep brain stimulation.