Is it worth training throw-ins properly at U13?

Yes, I think it is worth training the moment after the throw-in. The technique is only the restart. The football problem is how the thrower and nearby players create a second and third option once the first pass is made. You can build it into a normal 5v5 or 6v6 game by restarting every ball out with a throw. For a short block, ask the attacking team whether they can keep possession for three passes after the restart. That quickly exposes whether everyone is standing still or whether somebody is moving to support the receiver. I’d keep the movements flexible. One comes short, one moves beyond, another balances behind the ball is enough of a picture. At U13, the players are ready to understand that the throw-in is not finished when the ball leaves the thrower’s hands.