That should never never have happened! I would dump the behaviourist they are making excuse for the dog!
You need to get a trainer! The dog should not have gone beyond the yard! The dog should have stayed if you told it to and the dog should have come back when you called it!
Now the dog has a bite history not good and you did that!. Now you "know" this dog cannot be trusted around children!
A GSD is not a joke dog! I got my clock cleaned by mine! And I was an experienced dog owner that did real dogs! Even still I had to step up my game and turn his "issues" around. The only one that ever got bit was me (and my other Dominant Male dog) but my GSD issues did not involve outsiders!
I stepped up my game and today my guy is safe in public and I am very very proud of him.
So.."you" need to decide are you gonna "Run with the big Dawgs or Stay on the porch?"