Do you have a trainer already involved?
Personally I would show the inlaws that you're actively working on the situation with a professional and also completely seperate Ike from your niece.
Put yourself in their shoes, their daughter has been attacked twice by your dog, even though he's only 6 months old. I'd be pretty ticked off as well, proving to them that you are working on the situation should allay their fears and if he is completely seperated from their daughter then they have nothing to complain about as she's safe.
Jasmine - Female Miniature Poodle - born Aug 15, 2010
Loker Delgado Von Stalworth - Male GSD - born Jan 26, 2012
Koda & Zazu - 5 year old male cats
Alex - Male Cocker Spaniel (rescue) - RIP Cuddlebug 2007-2010