The book I am looking for is a YA about a young girl that hides in her father’s wagon when he heads to California for the gold rush. She brings a small pail of milk and it churns into butter. At some point they meet up with a lady and possibly another child, the dad gets sick or injured, maybe he dies. They go to San Francisco? – their house is on a hill? – and start baking and selling hardtack for the other miners. The book describes ships sitting in the bay empty due to the sailors abandoning them to look for gold. One night several of the ships catch fire. I read it in the mid eighties, definitely before 1990.