The Singing Tree by Kate Seredy

From chapters.ca:
Life on the Hungarian plains us changing quickly for Jancsi and his cousin Kate. Father has given Jansci permission to be in charge of his own herd and Katehas begun to think of going to dances. Then, when Hungary must send troops to fight in the great war and Jancsi's father is called to battle, the two cousins must grow up all the sooner.

While this is a sequel, I haven't read the first book. The Singing Tree stands on its own.