True a complaint can be filed against a player for cheating. If the kid is a really bad cheater I would not discourage filing a protest. But without corroborating witnesses and if it's just for (perceived from the sidelines) bad line calls that in itself will not be the sole source of sanctions. I'm sure your example cheater was placed in his position because of what he did in front of officials, not just those on the sidelines. If he was being punished for what he did outside the view of officials its safe to say it was for more than linecalling alone. The USTA is not going to punish a child due to a letter writing campaign for line calls alone.