In my opinion parents are always the one responsible for their children, whether it be education, physical safety, mental, or emotional safety. Its hard to get all those things covered as a parents and that is where i think that computer software and other things similar are a very nice and in some ways necessary thing to have, but ultimately I believe it is the parents job to make sure that their child is being taken care of. Including making sure the software that is on the computer is appropriate.
It should be the responsibility of parents, not computer manufacturers, to protect their children online. This may require the installation of certain software programs, but it should ultimately be the parent's job to restrict what children see and don't see while online. Internet use should be treated no differently than any other social interaction.
I believe parents should protect their children on the Internet. If they do that through software they shouldn't expect the easy way out. You'll have to test the software and make sure it meets your standards. Software is simply a tool, it is not the answer. Parents should consider this their duty, they shouldn't lay the responsibility on code.
I think a child's parents should ultimately be responsible for the safety of their child online, software can easily by bypassed or fail to do it's job, I think a parent concerned about their child should be actively involved in their Internet activity and allowing and disallowing certain actions the child can do on it.