Youth increasingly use AI companions, and parents are the primary mediators of that use. How effective that mediation can be depends on whether parents are aware of how adolescents actually use these systems and what risks and benefits such use carries; nevertheless, prior work has only studied these demographic groups in isolation, and existing taxonomies attend almost entirely to risk. We analyze 1,628 Reddit posts about youth AI companion use from parenting and teenager communities (2023--2026); develop a codebook covering modes of use, risks, benefits, and parental mediation; and apply it at corpus scale with an LLM. The two communities yield divergent accounts. Teenagers most often discuss receipt of emotional support from AI companions (31% of teenager posts vs. 19% of parenting posts), whereas parents most often discuss teenage use of AI companions for romantic and sexual interaction (36% vs. 25%). Teenagers are not unaware of other risks, however; indeed, attachment and dependence is the risk they raise most (19%), close to the parental rate (16%). Teenagers also describe benefits that risk-centered taxonomies do not capture and parents rarely mention, most notably emotional support (27% vs. 5%). We argue these differences track what a given kind of use makes visible to someone outside the conversation. Chatting with a companion for hours every night leaves a trace beyond the chat itself; sexting with a character stands out when a parent reads the log; venting about a fight with a friend does neither, since it looks like any other conversation. The first surfaces as dependence, the second as sexual content, and the third as emotional support, which is the one parents most often miss. Parental guidance and system design should attend to use cases that reach parents by neither route, emotional support foremost among them.

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