In general, I believe it’s best to have a similar role to what we call a secondary service provider, assuming the family has a general early interventionist (sometimes called a developmental specialist or therapist). It could also be possible, theoretically, for you to be the primary service provider, but we wouldn’t want two people trying to function as primaries. That takes away many of the advantages of a primary-service-provider approach. As secondary, you would make joint home visits with the primary (usually a general early interventionist).
The focus of those meetings would be on your consultation with the family. The primary is there to ask additional questions, make sure the family understands your ideas, prompt the family to assess the feasibility of your suggestions and to ask for alternatives if necessary, and pay attention to your suggestions, because he or she will be supporting the family in carrying them out.
As you know only too well, a second and hugely important role is to encourage the family to become proficient in the communication mode they’ve chosen. Ask them to think about what it would be like for a hearing infant or toddler to have parents who could say only more, sit, and cookie. If the early interventionists is any good, he or she knows a number of signs. But nowadays parents can learn to sign in a variety of ways. I would expect that, in addition to teaching them some signs, you also counsel them on ways they can learn without you (e.g., Internet sites, community college classes). Also, you can tell them about the order of signs very young children learn, which is different from the first spoken words hearing children learn. So your expertise is vital, but not weekly—if there’s a general early interventionist—and not just as a signing teacher.
Photo from Special Education Department, Illinois State University