6 years old is a bit young because at that age, they need a reason to engage with anything.
Games might be a place to start, but a kid will think "tablet is easier". Same with music or videos.
Art is another place to start, but even then, why computers?
You need to wait until they have an idea or goal they want to accomplish, then maybe use it as an illustrative tool. In most cases for kids though, a PC is unnecessary with phones and tablets being so prevalent with an easier to grasp UI/UX.