I was in the same boat... x 2. Our oldest is 15yrs old and he's currently 5'9, followed by our 13yr old daughter at 5'3. His first bike was a Motobecanne hybrid and my daughter out grew the Scott JR24 Speedster she was riding for a couple years. I lucked out on eBay and found some great NOS Felt carbon framesets this past winter; a FR Team frameset for my son and a ZW for my daughter. Both bikes ended up with a mix of 105 R7000 and Praxis (my go to cranks) and since both builds required disc brakes, I went with Juin Tech F1 calipers which are hydraulic-cable pulled brakes. For the wheels, nothing too expensive and opted for the DT Swiss R500 105, 32 spokes F/R and tubeless ready. Last I checked, Velomine still has them on sale too.