Kickr Bike Compatability

Hey everyone. New to Zwift; loving it. I went full boat and got the kickr 2020 and put on an old bike with a 9 speed drive train. Got a 9 speed cassette and a new chain and off I went. My little 7yo cousin loves it and I started trying to investigate what kind bike I can buy him to be able to just hot swap bikes. I might buy a second trainer too.

So, does anyone have any reccomendations on what options I might have drivetrain/bike wise that I’d be able to hot swap onto the trainer without having to change cassettes etc. I’m not a bike person until now apparently so I’m not too sure on compatabilities and what not.

Thanks for your time.