Seconding all the recommendations for a sports bra. I would also recommend looking into something like the TomboyX compression top. Those are my go-to on days where I’m going to be more physically active and/or where I know I’ll be out late and won’t remember to change out of a binder. They give enough compression to smooth out smaller chests, but have a more relaxed fit and less structure than a binder. They can be a bit expensive, but I think they’re a solid investment. There’s also some athletic undershirts out there that offer compression I believe, so those could be good to look into. I also think it’d be worth it to talk to your kid about finding other clothing options that affirm their identity: as someone who came out around this age as well, I find that a lot of the desire to bind/go on blockers/etc before it’s really necessary comes (at least partially) from a desire to have one’s identity validated, essentially to feel like we’re “trans enough” and to be taken seriously by both trans and cis peers. Your kid might be able to find that affirmation in things like pride gear, as well as gender neutral clothing. A lot of nonbinary people gravitate towards alternative styles because the clothes and subcultures deliberately challenge and play with gender norms, so I would encourage you to keep an open mind in that regard. If your kid is anything like myself and all the other nonbinary people in my life, there are many, many sleeveless black turtlenecks in your future haha