This paper presents a system architecture for synchronous mobile Computer-Supported Collaborative Learning (mCSCL). The paper builds on the notion of content-independent mCSCL by adding the technology support designed according to the contemporary design principles to support young learners. The specifics of the presented architecture are the authoring application that allows for reuse of interactive web-based educational application modules named "widgets". Widgets communicate via the Group Communication module to deliver group work educational experiences. The Group Communication module consists of the three sub-modules: Message Forwarding Sub-module, Activity Management Sub-module and History Management Sub-module. The three sub-modules are designed to deliver synchronous mCSCL experiences by distributing communication messages to the subscribed groups and their members in a timely manner and by ensuring possible state reconstruction necessary for smooth learner transition between widgets and devices. Such an approach adds to the robustness of synchronous communication which is demonstrated by utilizing the system and the developed widgets in two case studies where early primary school children learn Mathematics in pairs and triplets. The presented system is able to deliver synchronous mCSCL activities including pairs and triplets in a dynamic early primary school environment.