oh wow, I just had a great idea!!

this does not involve bikes.
you have other kids, right?
get their favorite snack, gum drops, cashews, whatever.
and play a game.
Whoever puts on a baseball cap gets a treat.
kids all put caps on, get treats.
whoever puts a scarf on.. gets a treat
kids all put scarves on, get treats.
Whoever puts on a bike helmet, even for a second,
gets a treat!

then focus on having her wear it for more than a second.
if i can train my cat to open and close doors, you can do the same with your kid!
good luck!