In analyzing the parent/child relationship in Romeo and Juliet, you might consider focusing on the degree to which Shakespeare establishes parenthood and/or childhood as a role to be played. From the party where the young play at adulthood and romance to the nurse and friar who stand in as parents for the young lovers, the play is almost a study in caricatures. Your introduction and thesis could spotlight the efficacy of the different "parents" in their intended roles. Juliet's mother does not even know her own daughter's age; and yet, the nurse does, marking time by remembering the tragic loss of her own daughter (Act I, scene iii). When Nurse states that "Susan is with God; she was too good for me," there is an eery hint of foreshadowing.
Language could also become a focus of your thesis as you identify ways in which Shakespeare labels and describes the different parental roles. "Nurse" may be an endearing title given to Juliet's wet-nurse, but the reality of nursing a child and nurturing her into young adulthood plays dramatically against the distant, political nature of Lady Capulet and her concern that Juliet be suitably married.
The thesis statement is going to guide the introduction. I think that an introduction about the child/parent relationship in Romeo and Juliet can focus on how intolerance does not work in such an association. Shakespeare demonstrates that the parent/child relationship fails when there is a lack of tolerance and acceptance of parents regarding their children.
If this is where your thesis lies, your introduction can talk about how the tragedy of the young lovers is established because of the war between their families. This war is triggered by the parents. Shakespeare suggests that when parents display intolerance and obstinacy regarding their children's insistences, there is a greater propensity for destruction and pain. An introduction for this can talk about how different elements in the drama support this. From the Montagues' refusal to understand through open dialogue what is wrong with their son, to the feud in Verona, to Lord and Lady Capulet essentially disowning Juliet, instances of intolerance and dogmatism ruin the parent/child relationship. I think that it might make sense to craft an introduction arguing that Romeo and Juliet asserts that when there is not an open tolerance towards children, parents invariably lose them.