When I got done with my GE interview I asked how long until I get my KTN number and he said that you get your KTN number right away. It's in the upper right hand corner. Membership ID/PASSID on the TTP dashboard. So when I applied for my kids recently I put that number in the SW reservation and they haven't had their interview yet. I don't fly until March, but I am thinking that Precheck will show up on their boarding pass.
In other strange GE/Precheck news, my husband has applied for GE, but hasn't been conditionally approved yet. I never put a number in his profile for SW, but yet when we fly a few weeks ago he had PreCheck.