I'm not too knowedgeable about yuri-themed media, but I can offer suggestions about
good things to offer to young girls.
Anime: Princess 9, hands down. It's cheesey, there's not really any
yuri and it basks in the stereotypical characters of the sports genre, but ALL
the primaries are girls, and they all learn to find the inner strength
to brush off the boys' ridicule and win.
Manga: My manga knowledge isn't quite what it could be, but I can suggest several
american comics -- Courageous Princess, Meridian, and Leave it to Chance.
In Courageous Princess (Rod Espinoza, Antarctic Press), a princess is
kidnapped by a dragon and must save herself when she realizes no
one is coming to rescue her.
Meridian (Barbara Kesel & others, CrossGen) is an epic fantasy where
Sephie must save her kingdom from her evil uncle, who has
killed Sephie's father and siezed the throne.
In Leave it to Chance, Chance solves supernatural mysteries
wth her pet dragon. It only ran 13 issues, but it's very good.
Book: Dealing With Dragons, by Patricia Wrede, Part 1 of the Enchanted
Forest Chronicles. Cimerone is a very bright, practical princess, and as
such chafes under the restrictions placed on her by her extremely
proper parents. Faced with an intolerable arranged marriage, Cimerone
does the only thing she can -- she runs away to work for a dragon.
This was my all-time favorite book grouping up.