My experience with young kids (12u and under):
Make a game out of it. How many line drives can you hit out of 10, while using proper mechanics? How many balls in a row can you throw back and forth with out dropping. Who can pick up the most balls, while cleaning up? Stuff like that. If they are very competitive they will beg you for more "games" aka practice!
You may have to be innovative with your practices, more frequent changes, with less lengths of time. That would be hard for me, when I get focused, I could spend hours doing something repetitive, but most kids aren't built like that.
You may also consider some "school" setting, type focus drills. My dd was in a program at school, I would use the same principles on the field. Google how to get students to focus at school?
One thing, I refuse to do, is reward a good on field performance. For instance, I would never say, "since you did good today, you get an ice cream cone." To me, kids do good for themselves, not for an ice cream cone....as a parent. Although, my dd has a coach, that will every once in a while, stop at the gas station and pick up a few bags of candy. Then, at the end of practice, reward a couple kids based on hustle or hard work. Not every practice, so they better work hard every practice, because they never know when they are going to get rewarded!
Good luck!