Purpose Volunteers at child helplines play an important role in providing support for children, so keeping them satisfied during encounters is crucial to continue helping children. The purpose of this study is to understand how children’s perceptions of instrumental and emotional support (partner effects) influence volunteer encounter satisfaction, and whether this effect is moderated by a volunteer’s previous encounter experience and levels of interpersonal and service-offering adaptiveness. Design/methodology/approach The sample consisted of 377 dyads of 116 volunteers and 377 children from online service encounters at a child helpline. Questionnaires were used to measure satisfaction, support and volunteer adaptiveness. A multilevel model was estimated to test the hypothesized moderation effects. Findings This study revealed that the instrumental support partner effect positively influenced volunteer encounter satisfaction. This relationship was stronger when the previous encounter was less satisfying or for volunteers with higher interpersonal, but not higher service-offering, adaptiveness. Negative effects on the relationship between the emotional support partner effect and volunteer encounter satisfaction were found after a less satisfying previous encounter or for volunteers with higher interpersonal adaptiveness. Originality/value This study contributes to the services and volunteerism literature by providing a unique perspective on the interpersonal influence between volunteers and children during service encounters. In the context of child helplines, this paper illustrates how volunteer encounter satisfaction is a function of the intricate interplay between children’s perceptions of the service encounter and volunteers’ perceptions of previous experiences and their adaptiveness.