Avatar: The Last Airbender. I could rave about it all day.
Also Gravity Falls like @swordgeek@lemmy.ca said.
I never watched Justice League much as a kid, but my wife re-introduced it to me as an adult and it's a pretty great mix of kid-friendly superhero stories with fairly mature themes and plotlines.
Additionally, a less awkward term than "kidult" is just "family entertainment". Shows/movies/books that can be enjoyed by a wide spectrum of ages.