What you need to do is limit your umpiring to all turf fields. Lakepoint comes to mind. Near impossible to draw a line in the turf.
But seriously, not sure what an umpire is to do. As for the instance in the clip, that pitch looked really far outside, so I can't blame the kid completely (although I would blame my kid if he did that on a bad call). Six instances in the last 12 months does not seem like a huge amount given what I assume is your workload, so it doesn't sound like it is trending and would need to be pro-actively addressed during each game. I also wonder how grandiose each instance was. The clip was pretty bad and I (a non-umpire) could see myself ejecting the kid the moment he looked back at me - let him draw a line and walk away, but the condescending look probably gets him tossed.
Those are just my early morning thoughts. What I really wanted to do was wish you the best of luck in dealing with these types of issues. Hopefully some others will actually provide some true insight, but please let this serve as my appreciation for umpires and what is oftentimes a truly thankless job.