Hey there!
a) I know there's scene in W/T Season 3 Y'all where that happens, but you said you've read that so it's probably not it lol.
Also, with some of the scoobies being turned into kids, there's
Kids - Tara is turned into a 3 year old (with some of the others) with lots of cuteness and Willow taking care of her.
b) No flowers, but if you haven't read Shel's entry to the fanfic comp,
The Santa Line, the first bit is about them meeting when they're little
Catching Sunflowers In Bloom has them younger, although not little kids, and hey, that fic's full of flowers
c) When Hate Meets Love? Willow is a self-proclaimed 'bad-ass' and Faith is one of her best friends.
And...hmm, well in
Altered States, W/F are friend, don't know if they're 'bad girls' as such... It has a sequel too,
Love Is Not Always Easyd) The ones I know that are canonical and not AU are:
First Meeting by Quirky Writer, which has Tara come in in season 2 to replace Oz.
The link is for the old board, because I'm pretty sure it's not on this one. If we're not allowed link to the old board, mods, really sorry, let me know and I'll change this to a PM.
Wave by LonelyTara, starts at the Halloween episode in season 2 where Willow meets Tara, really fantastic story so far.