've run into the wait "kksfbc child completion" a few times over the past but found very little written about it. I don't have an explanation, but I have something that might be as good - a way to reproduce it. By being able to reproduce I at least test theories about it.
I ran a tight loop of
insert into t value(1);
See anything strange? I put "value" instead of "values" so the insert errored out. Thus causing parsing every execute. So this wait seems to be related to parsing. Could run further tests by correcting and adding a different litteral value to the insert to see what kind of waits might result.
Another tell tale sign that there was an error was that I also recieved "" which is a sign of an error in the sql statements that the application is attempting to execute.