Just know that running on a treadmill is different than running on a track/road/trail. The belt is doing some of the work for you. But as long as the teen is doing plenty of training outside or on an actual track, a treadmill is a great augmentation.
Also the strength training with Peloton, they are very good. What someone gets out of them is how much they put into them. Basically, the person has to be self-motivated, not someone who needs to be motivated by peers training in person with them and a coach in person telling them to not slack off.