If any forward motion pre-snap is illegal, how can you go from the huddle to the line before any play? Every lineman would be guilty of illegal motion on every play, or you don't have any linemen, so you have illegal formations.
Having done many youth league games over the years, 90% of the time when a coach says "Hey, we want do this trick play we saw on the internet" the trick play violates any number of procedural rules or tries to subvert the intent of actually playing football (*cough*wrong ball play*cough*). As such, any description is met with significant skepticism as to whether or not it aligns with the rules and some of my fellow officials tend to translate that into "We're not running any silly trick plays on my field!", which defeats the point of letting kids have fun playing a game.
I can't say for certain that the play you ran was legal, as I'd have to actually see the play, nor can I say that the official who flagged the play was wrong in his interpretation, because I didn't hear his words, but illegal motion is only a problem *at* the snap, not before the snap.
