Yay! Look, I can actually provide real information for once!
Love You Two, by Maria Pallotta-Chiarolli
for Young Adults (the protagonist is 16)
Excerpt from the first customer review on amazon.com: "Love You Two is the first novel from Australian academic and researcher Maria Pallotta-Chiarolli. When an average teenage girl accidentally discovers that her mother is polyamorous - she has a boyfriend in addition to her husband, and all three partners are open and honest about the situation - her rage and confusion drive her into a tale of self-discovery and personal growth, and triggers a chain reaction that brings down the many walls of secrecy among her family and friends."
I should also point out that I haven't read it, but I did hear about the book from the author herself. And an attempt to find books on polyamory for younger children on Amazon brought up books that argue against same-sex marriage and polyamory...my brain feels dirty....