You've been given the rule citation: ASA 8-4-H. The rule is fairly lengthy, so rather than typing it in, I'd suggest you get a copy of last year's rule book (the rule was not changed) - which any travel coach should have kicking around in the bottom of his equipment bag, probably unread
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In the scenario you posted, as Mike said, the runner would be returned to 3B assuming the ball did not go out of play on the overthrow. If the ball went out of play, the runner would be awarded home.
Under ASA's 10U rules, the runner can score only on:
1) a batted ball
2) a base on balls or hit batter with the bases full
3) on an awarded base when the ball goes out of play or illegal pitch.