I hear every so often the expression (in regards to pitching) "You can't teach speed". I disagree, especially when the kid is taught properly from day one. However, a 14u kid with accuracy problems throwing 50 mph is going to be a steep challenge.
What you teach should be smooth, proper mechanics combined with quality speed drills to learn "quickness" an intensity. Just throwing hard isn't enough, because without great form, accuracy will never come. A wild college pitcher throwing 70 mph is FAR worse than a pinpoint pitcher throwing 58 - 60. The wild pitcher will try to take something off to improve accuracy, but the root of the problem is still there - ingrained bad habits resulting in poor mechanics. Smooth, repeatable quickness and explosive intensity.
So... what's better at 10u? EVERYTHING is important, but if her mechanics, form and accuracy go in the toilet when she starts trying to throw faster pitches, you better back up a few steps and fix that BEFORE trying to develop the next 10u speed phenom! The great pitchers have one thing in common - command of their pitches. They can throw their changeup the VERY FIRST pitch of a game without even batting an eye. They can fearlessly throw tight inside across a slappers knuckles, knowing exactly where the pitch is going on release.
Watching a young kid with pinpoint accuracy and a tremendous CONCEALED changeup is far more impressive to me than a kid who throws much faster. Kids trying to throw above their abilities is like watching a race where cars are always crashing out of control into the guardrails. Batters will ALWAYS catch up to a fast pitcher with poor control. But a kid throwing with age-appropriate speed who is well taught has a much better chance of advancing to the college ranks.