Coaching minis was the most draining experience I've ever had, so I stand with you in solidarity. This is what I found worked for me:
1. Have as much structure to your practice as possible. I used to make a minute-by-minute practice plan and run practice in 15 minute blocks. Every 15 minutes, we'd refocus and do something different or take a water break. The littles can't focus for much longer than 15 minutes, so it's best to get ahead of them and change activities before they lose their minds. If a something needed more than 15 minutes of dedicated time, I'd do that activity in two sections.
2. Make everything a game and change it up constantly. One of our favorites was the princess puzzle game - we'd put together a huge foam puzzle with one piece for each section that we completed successfully. The prize was a 30 second dance party. Simple.
3. Perform for others. Take any opportunity to get in front of an audience and perform. It could be a section, a walk-thru, a stunt section, or a full out. If they have an audience, I promise they'll behave 100x better and you'll get so much more out of them.
4. A run-thru is a run-thru, no matter how it looks. At the beginning stages, when a mini routine is just coming together, all you'll care about is whether they know where they're supposed to go. We'd walk through the routine with music 4 or 5 times a practice, but we'd do it in different silly ways to keep them from getting bored. We may do one walk-thru as robots, one as monsters, etc.
5. Hugs and high-fives all the time. Let them see that you love them and you're proud of them. It may have absolutely no visible result, but the kids will feel loved and supported, which is the most important thing. This is something I didn't do enough in my year of coaching minis, and I regret it all the time. The kids say they miss me and want me back, but that's only because they won, not because I was actually very nice. :(