'Taking part' in conversations requires different activities from the interactants depending on the kind of conversation. This article investigates co-constructions in oral peer group discussions of elementary school children from grades 2 to 6 (7-12 years old). Although dissent is the starting point of argumentations, negotiating processes in oral argumentations are often co-constructed by two or more speakers on different levels, including consensual contexts. Co-constructions presuppose that the second speakers recognize structures and expectations based on the turn of the first speaker and that they are able to complete or expand these structures. Therefore, co-constructions can be understood as an indicator for oral skills and as a key site of 'taking part' in small group discussions. The article will discuss two different kinds of co-constructions (morpho-syntactical and argumentative-structural) based on 60 transcripts from a bigger corpus of 180 peer discussions. The analysis will show that these co-constructions can be understood as synchronizations of thinking and acting and to what extent they are an indicator of oral skills and play an important role in cooperative learning settings. The results are relevant in school contexts when it comes to assess oral argumentation of students. For teachers, they are helpful to elicit requirements for children's argumentation skills and to design tasks conducive to learn to argue and develop assessment tools accordingly.