The number of the child varies and this only seems to happen when I am running too many threads that are starting too many parallel child activities. I'm trying to get our application to successfully run more threads, so I'm trying to understand why the child is getting lost.
I see there are hot fixes related to this error for 5.4 SP1 and 5.4 SP2. The error appears to occur when two child requestors get generated with the same ID. Please open a Support Request and an engineer can work with you further to determine if the hot fix will resolve your issue.