Teens are intensive users of social media recommender systems yet remain largely excluded from platform design and governance. The EU Digital Services Act (DSA) has the potential to address this through its call for stakeholder consultation in risk assessment and mitigation. To demonstrate how teens can be meaningfully engaged with as stakeholders, and to understand their experiences of algorithmically curated content, we conducted a participatory engagement study with 87 teens using scenario-based think-pair-share discussions. Findings reveal an ongoing negotiation between benefits, risks, and harms, as teens attempt to influence algorithms while managing the emotional impact of harmful content and excessive use. We contribute: (1) an account of trade-offs between personalisation, harm, and control; (2) teen-informed design recommendations, including safeguards, low-effort feedback, and screen-time tools; and (3) a participatory engagement method. This research reframes teen recommender system use as negotiated agency rather than passive content consumption and offers actionable guidance for DSA related stakeholder engagement with teens.