450 marbles (or gumballs, or beads, or other small items)
Before recounting this story, hold up a marble and tell the group that it represents Abinadi. Ask the group how many people they think were converted through Abinadi's efforts. Write their guesses on the blackboard. Then recount for them Abinadi's short history as recorded in Mosiah 11-17.
In summary, Abinadi, in fervent love and obedience to the Lord, boldly testified against wicked King Noah and his priests. Abinadi was ridiculed, abused, and even eventually condemned to death by fire at their hands, but Abinadi would not withhold his testimony. He died, probably thinking that not one soul had been changed through his efforts. But we all know how a penitent Alma (the Elder) allowed Abinadi's words to penetrate his soul. Alma left these wicked people and converted many.
At this time, uncover a jar holding 450 marbles (see Mosiah 18:35) and dump them out on a tablecloth or blanket on the floor. Demonstrate to them that one tiny obedient person (one marble) has the power to change hundreds of lives, without ever knowing it.
Remind them that this was just the beginning of Abinadi's effect. Alma and his people converted many more. And Alma's son (Alma the Younger) and his friends eventually even served among their enemies the Lamanites and converted many of them. (You could dump even more marbles on the pile as you explain the exponential effect of one faithful soul.