I love the psychology of coaches who yell out 'throw strikes' during a game. The hitter knows the next pitch is likely going slowly right down main street.
I would randomly yell out 'curveball', then watch the hitter start wiggling around, prior to their strikeout.
Not to mention that of course a 10U girl is trying to "throw strikes". That's the single-most focus. Telling her to do what she's trying desperately to do is kind of counter-productive.
Here's our 10U pitching core's stats. Not sure if it's average for 10U (travel - B) or not, but might be of slight interest to cnardone.