I have little girls that like to open up my front door when people knock. They know the rules, they know they aren't supposed to, but they occasionally get rather excited when they see someone they know coming, and open it anyway. It's been a PITA since I got Lyka. I installed a latch at the very top of the door like they have in hotel rooms so they can't reach the lock to unlock it. You may do that if your 11yr old can't reach?.
Last week, I was on my front porch in the morning. My daughters were asleep when I went out. One woke up and opened the front door to see where I was right as a jogger was running by with her dogs. Lyka saw them through our large front window, and as soon as my daughter cracked the door, Lyka was off and running. Luckily, she just sniffed the dogs and I was able to grab her before she bite (she is a rescue, and highly dog aggressive), and got her back inside. Now I have a baby gate between my front door and the security screen. That way, if somehow they get past the lock, at least the gate will slow Lyka down in getting out. She isn't a jumper, so she just kinda lunges at the baby gate. It's not a perfect solution, obviously, but it helps.
As for the bite, I'm no expert, but it sounds like your GSD was playing and didn't intent to harm. But, it's still a recipe for disaster. Most definitely get a trainer in to help you. Lyka is improving by leap in bounds in the short time I have been working her with the trainer.
Hopefully your dog is UTD on all shots? Have the neighbors (adults) had interactions with you and your GSD before? I would be much more open to letting it slide if it were my child if I knew the owners were responsible and took time to train rather than it being a negligent owner.