Throw-in by benches

[Chess Ref]

The ball goes OOB in front of the bench. There is like 2 feet between sideline and the chairs. I have been telling the kids they need to give the thrower 3 feet or so of room to throw it in. I had a kid who started out 3 feet away then gradually moved in closer and closer during the throw in. i can't find any rule reference to support my position. Is my position supportable by rule or Am I making stuff up here ?

Thanks for the help and feedback.