Hmm...did you ask the shop the question about painting and the warranty. Are they decals you can peal off or can you cover them with sort of other design? Know anybody with an artistic knack who can add on to the flowers to make them look more "mature?"
Beyond those suggestions, I can only say kids will be kids and they'll be fussy about simple things because that's something we all go through.
Oh, one more somewhat sneaky and tough to pull off suggestion: convince her that it's cool to ride a flowery bike. Remember the scene in
Billy Madison where Adam Sandler convinces all the other kids it's cool to pee your pants so the kid who actually did wouldn't be so embarrassed?