Liability waivers have not, to my knowledge, ever protected anyone in the medical community/profession from being sued....adult OR child.
People want to make "their" choice, and then hold others accountable when things go wrong...and it doesn't help that they are egged on in this endeavor by a bevy of lawyers.
They help frequently, but are not foolproof. Part of the problem is how they're done. Usually the patient has a long document with fine print thrust in front of them, with a "sign this" order from a receptionist or nurse. It's easy for a to court to find that the patient never really understood the document, and thus never really entered into the "contract". This could be fixed by a brief videotaped "waiver interview", with a nurse or other professional asking the patient "Do you understand that X may happen? Do you understand that if X happens, you (or your child) will have no right to sue the doctor for damages?"
The biggest area where they don't work is the patient being rushed into surgery, who clearly doesn't have a choice as to whether to have the surgery done, and thus just signs anything. Such patients will not be found by any reasonable court to have truly consented to the contents.