Lawsuit

You probably need an attorney to do that for you. Just stating that the child is of age may not be enough to get the parents out of the lawsuit. There may be other factors involved. For example, in Florida, we have the dangerous instrument doctrine which makes the owner of a vehicle responsible for injuries caused in a motor vehicle accident even when someone else is driving. If the child is legally an adult and causes an accident while operating a vehicle owned by the parents then it would be proper for the parents to be included in the lawsuit. There are many different scenarios that could result in parents being included in a lawsuit so you should have an attorney advise you. Check the Avvo directory for help finding one.

There isn't enough information in the question to provide a meaningful answer. An attorney would need to know at least the following: What court was the case filed in? Was it a federal or state court? Was a complaint filed, or just a summons? If there was a complaint, what was the claim, and in particular, what was the claim against the parents?

This question is posted in the Intellectual Property section, so I speculate that it might be applicable to a copyright infringement suit by a source of music against someone who has downloaded and distributed recordings online. If so, it there an allegation that the parent is vicariously liable?

Exactly when was the case filed? Once a lawsuit starts, a bunch of deadlines become applicable, and if they are missed, then either the party automatically loses, or at least that party has a heavy burden or showing why the deadline was missed. It is important to get a copy of the court papers to a lawyer as soon as possible to avoid such difficulties, which at worst could leave the defendant with a judgment against him, and at best, involve additional work and hence expense to restitch the 9 stitches that wouldn't have been needed if there were a stitch in time.