Ahh, the joy's of youth baseball 
. Fast is fast so try not to worry about timing a 10 year old. On the team I coached, certain kids had a green light at all times (short base paths, inexperienced catchers, and quite a few wild pitchers). I didn't need a stopwatch to tell me that. However, my experience with youth baseball is that the "top" teams aren't usually that fast anyway. You don't need that skill to hit 205 ft fly-balls. It makes me cringe when I hear youth coaches using "measurables" to grade out kids, especially at 10u.
In all seriousness, take Goose's post to heart. "Stick around and read lots of archived threads" You'll probably find that most of us started out in your shoes.