Your novel is exactly what you thought it was: a solid MG. Your protagonist’s age and grade are right for tween readers. The 7 to 10 age range you cite sounds young to me, more befitting of the chapter book category. MG is commonly considered to start at age 8, extending up to age 12, at which point the YA category kicks in. Your character is thirteen years old, but she will appeal to older tweens, perhaps 9/10 to 12, as most kids like to read ‘up’. Because young readers tend to defy strict categorization, there’s a lot of wiggle room on the top and bottom of the age ranges, but here’s a breakdown that can serve as your general reference for teen/tween categories:
Chapter book: Ages 7 to 10; fully developed characters and longer text (roughly 100 or more pages) broken into chapters; may include decorative ornaments and/or limited black-and-white illustration.
Middle grade fiction: Ages 8 to 12; longer text, may or may not be illustrated.
Teen fiction: Ages 12 and up; more sophisticated plots, characters, and subjects.