Dear Kerrie:

Your question revolves around social interactions in young infants and toddlers and ask for suggestions of authors/researchers who elaborated on the issue on social interactions. I wonder whether you are acquainted with Piaget's (1932) theory, namely his book "The moral judgment of the child". In this book Piaget theorized about two types of social interactions: vertical social interactions, a child-adult interaction, for example, and horizontal social interactions, or interactions between peers, for instance, a child-child interaction. Piaget argued and showed that vertical interactions are likely to lead to a heteronomous morality, a morality guided by the idea of fear, constraint, obedience, and unilateral respect, whereas horizontal interactions are likely to lead to an autonomous morality, a morality based on the idea of equality, cooperation, and mutual respect. Note, however, that in in the book referred to above, participants are children from 4 to 11.  Even so, is it feasible that Piaget's theorizing about social interactions provides you with a theoretical framework for you to ground theoretically your research. 

In 1987, Judy Dunn wrote a book, titled "The beginnings of social understanding". As is visible by its title, the book is closer to the ages you are interested in than Piaget's. I think that you can profit a lot from reading both. Please, forget this kind of advice if you know both books.

Best regards.