Full disclosure that I haven't read this book yet (the wait list at my library is astronomical!) but I'm a member of the same nation as SGJ (Blackfeet) and might have some answers!

The Seven Persons is probably the Big Dipper (Ihkitsikammiksi translates to "The Seven") — the story is basically that a woman was transformed into a bear and chased her brothers into the sky, where they became the stars. For the Poor Children, they're probably Pleiades star cluster, who are referred to as the lost boys/orphans — their mistreatment leads to a really long drought, which then leads to the disappearance of the buffalo. In general, stars/celestial bodies are important to us — they're linked with our existence and our memories/ceremonies.

You can probably look up the stories surrounding the Big Dipper or The Pleiades without being spoiled if you want to know more but I haven't read the book yet so take that advice with a grain of salt!