Well, keep in mind the american government are the ones who promise not to kill kids at times. Then try them as an adult randomly.
EDIT: what i'm getting at is that america promises to not do things like suing 9 year old and not killing other kids most of the time. However, exceptions be considered, granted, and exceptions in this case can go **** themselves. Let's keep suing 9 year olds and keep trying kids as adults. How exactly do you sue a minor? Well, we can take lessons from when young murder suspects are tried as adults in murder cases. Unfortunately exceptions were made for when and must a court try a child as an adult. That's how you can sue a 9 year old.