I think it should be fine for her to watch Rhubarb, my DS started watching Superman, Batman and all Superhero things since he was younger than your DD. My DD is only 2 so doesn't quite understand what she was watching, she kept saying "Supermanlady"
DD is 5 and I'll be watching it on a Saturday afternoon with her (when it is repeated), though I'll be checking the episode on a Thursday night.
It was quite refreshing that the main characters are female - the newspaper boss, the sister / superhero government bod (very Frozen with sisters doing it for themselves!!), female superbaddie and obviously supergirl!! They handled the calling her supergirl instead of superwoman fairly well too.
I did find it a bit try hard for feminist credentials in places, they need to let the characters just develop instead of shouting this is for us women, but I guess it is one of the first female led shows for a while (with the exception of OITNB).
it was good but they laboured the point on the girl angle why is it when boys grow up they become MEN girls grow up they STILL get referred to as GIRL and that's okay apparently not demeaning at all they could have just passed off the name with no hysteria but they had to make a point <sigh>
and that's my ONLY grumble im shocked how good it is