The key is whether the play should've been made with "ordinary effort."
That is inherently a judgment call.
From the perspective of the hitter, if a ball I strike lands in the outfield grass, I'm expecting to be credited a hit unless they do in fact get me out. So I can see where dad was coming from.
But while you don't normally put out a guy with "ordinary effort", it sounds like the RF played in specifically with an eye towards making exactly this type of play -- to the point that the out could/should have been made with ordinary effort. A varsity player ought to be able to throw a ball 50' with accuracy, after all.
So I think you have sound reasoning for your decision. As an exercise of discretion, I probably would've given the kid a hit. Probably the only time I'd do an error on that play would be if the throw were made and clearly on time but the 1B just plain dropped the throw. But that's just my personal preference, it doesn't have to be yours.