I’d keep a middle-third throw-in very simple.
You need three things: one short option, one player creating movement beyond or away from the throw, and somebody protecting the space if possession turns over. The biggest problem I see is five players all coming towards the thrower and making the area more crowded.
I’d also give the thrower permission to go backwards. Retaining the ball and rebuilding is often better than forcing a throw down the line because everyone thinks a throw must move you forward.
Train them inside normal games. Whenever the ball goes out, restart with a throw rather than having a separate 15-minute throw-in drill.
At U14 I’m not looking for routines.
I want the thrower to scan before collecting the ball, teammates to create different heights, and the team to recognise when the safe backwards option gives them a better next action.