Congratulations to your daughter and her bike training, and kudos to you, doing good things for your child. My advice comes from that of a mom that raised cycling kids, and ran a program at their school that encouraged parents to leave the car at home by doing bike days and more.
It's really important that your child, in the clothing that you put on her, feels comfortable so that she will continue to ride her bike: your ultimate goal. For example, if absolutely no one at school is wearing a safety vest to ride their bike to school and you force her to, she may get teased and won't be wanting to ride that bike. If the highly visible waterproof jacket is non-breathable fabric and is sweaty and clammy to wear, same result. There's a delightful balance between safety/visibility, what your wallet can handle, and what will be acceptable by her peers.
If you can afford a new waterproof, breathable, highly visible and fashionable cycling jacket for every time that she changes sizes, great. But if you're like me, finding a brightly coloured jacket, with a few reflectors in the right spots will be enough to feel assured that she's visible. As she gets older and is riding alone at night, increase the number of reflectors and lights.
Don't allow a cross-strap bag; they create lousy balance, and back problems too. If you don't have racks and bags, then a decent-sized backpack is best for keeping arms free, and allowing the torso and head to turn for traffic-checking. Too big, and mobility becomes an issue. A place for a water bottle will start a great habit.
Check out this family's journal...pretty inspiring stuff:
Good luck to you and yours!