The Black Alabaster Box (Last Crystal Trilogy)

It had been quiet along the Santa Fe Trail for more than a year when the Stokes Company set out for California, the Willis family among them. A reluctant traveler, young Grace Willis longs for her fortunate, safe, and comfortable life at home. Just as she is learning to negotiate life in a wagon-train, Grace is kidnapped by fellow travellers and taken into Oklahoma Territory. She must decide if she will cave in to despair or muster the courage to run away and search for her parents. Grace finds help in unlikely places. She discovers that there really is such a thing as magic, and there are some things only a child can do.