In this paper, we present a socio-technical process designed to engage children in an ongoing urban design project-Streets for People-in Newcastle, UK. We translated urban design proposals developed by residents and the local authority to enable children to contribute ideas to the project. Our process comprised three stages: situated explorations and evidence gathering through digitally supported neighbourhood walks; issue mapping and peer-to-peer discussions using an online engagement platform; and face-to-face dialogue between children, residents, and the local authority through a 'Town Hall' event. We report insights gained through our engagement and show how our activities facilitated issue advocacy and the development of children's capacities, but also surfaced tensions around the agency of children in political processes. We reflect on the challenges of working in this space, and discuss wider implications for technology design and ethical questions that 'scaling up' such work might pose.