Sarah wanted to protect everyone from Barnabas, and at the same time, protect Barnabas from everyone else. She had to protect David, who was sneaking into the Old House and finding out more and more, so she tried to scare him off from a situation where he found out about Barnabas there and was discovered by him, and killed. So she tried to tell him what Barnabas was, asking him to keep quiet about I think, and stop going there. This also protected Barnabas, if David did as she asked.
I think Sarah wanted Vicki, or someone nice, one of the grown-ups, to find out how Barnabas came to be as he was, in context, to protect Barnabas. I think Sarah thought Vicki and then others she told about the experience would try to help Barnabas and not judge him too harshly, and stop him without destroying him. Sarah was just a little girl despite everything, frozen forever as a child, and she had no intricate master plan. It's not Sarah's fault that Vicki became embroiled in this whole witchcraft thing, and was sidelined in "gaol" while the vampire story unfolded. She's just one ghost, and doesn't control life and how it plays out.