My girl is seven and she thinks Sabine is the best thing ever! I would agree, but then go so far as to say that both female characters are strong, clear and (dare I say it) feminist(?)
Technically the characters in Rebels exist in a post-feminist context because, like you point out Gaelis, the competency and equality of the female characters are never questioned or commented upon with a lens to their sex. Within the context of the show Hera is lauded as being a superb pilot and leader by both female and male characters. The other male and female characters don't comment on how she's a great pilot "for being a woman" and the male or female characters don't question her ability to captain the Ghost because she's a lady. Neither does her under the radar relationship with Kanan define her as a person. She, and other characters, don't have to advocate for the female characters' rights because those rights aren't in question. That makes the show's worldview post-feminist because equality between the sexes is the norm, not an ideal being fought for.
That doesn't mean though that the show doesn't make feminist commentary. The only place I can think this happens explicitly is in the episode "Idiot's Array" where Hera has to contend with the advances of both Lando and Azmorigan. Lando tries to charm Hera but she doesn't buy into his wheedling compliments. Later Azmorigan tries to set Hera up as a slave/harem girl and Hera proceeds to incapacitate him before escaping on her own without the need for a rescue. Both of the male characters treat Hera in a patronizing manner (Lando less so, he's trying to seduce her) and she wins the day not by relying on other characters to help her but by figuring her way out of the bind that she's in. This lampshades the typical damsal in distress archetype and highlights how unwise it is to treat people using assumptions about who they are based on their sex or species (Azmorigan expects Hera to act like a Twi'lek dancer because she's a Twi'lek female).
Anyways.... As a father of an infant-heading-towards-toddler daughter I too am very aware of what caliber of media is being produced to entertain young women and inform them to cultural "norms." Back when I was a kid I can only remember one female character in cartoons, April O'neil from TMNT, and she was mostly relegated to the role of plot device, someone to be rescued. I'm very appreciative of the fact that Rebels has three different flavors of heroine (rebellious artist Sabine, dependable leader Hera, and wise warrior Ahsoka) who have rich (for a cartoon) backstories that respects them as people. It's rare that children's media has male and female characters who can serve as role models for children of both sex so I gotta applaud that Filoni (the director of Rebels) and crew created a show that caters to both young women and men.