I read that one in middle school and still remember it with affection.

Trixie is 13 in the early books. I remember that she seemed much older to me when I read them–around age 11, I think. I read the whole series, which was tricky because the library didn’t stock series fiction back then, and I didn’t have much allowance so buying books was hard to do. Finally got a bunch of them when a store near us was going out of business and I got them at something like 75% off.