I would, and have, told the player to go and get the ball. Have done this at U16 level, trying to remember if I’ve done it at U18. May even have done it in OA, but not sure.
Wouldn’t do it in every game, I’d “read the room” first, but it’s certainly more powerful than either not doing anything, or giving a yellow card.
But then I do have 28 years experience in the classroom as a teacher!
“You can go and get that” issued in a firm, but non shouty or aggressive tone, usually gets compliance - in part because they are shocked at being told what to do, they just get on and do it. “It will be a yellow card if you don’t “ would be my follow up if they don’t comply.
A high risk strategy- and I may cop some flack from my fellow refs - but for younger age groups, well worthwhile. All but the most knucklehead parent or coach will be appreciate and support your actions.
If they don’t get the ball, when issuing the card I would say: “I gave the chance to collect the ball, but you chose not to get it, so you chose to get a yellow card.”
As an aside, you do still get some youngsters who go and collect the ball when it’s not their team’s restart - say corner, goal kick, throw - when they do, I always publicly thank them, and insist the restart doesn’t happen until they are back in position.