Diane Duane's "Young Wizards" books

Because someone asked for recs of fantasy books for an 11-year-old.

There are a lot of things I like about Diane Duane's "Young Wizards" series, and I think I would have loved them when I was a kid. As an adult, the inconsistencies sort of drove me nuts.

Two kids, I think they are 13 and 11 at the beginning, become wizards. They have to save the world a lot. There are alien cultures and monsters and alternate universes and all the stuff I really like in sci-fi/fantasy. The kids also have good relationships with all the adults in their lives, so it's sort of the opposite of Harry Potter that way.

Here's the first one: