Have a look at the parental controls for Android and Apple, decide what you want to prevent (eg usage at night, downloading apps without permission, taking photos, using a web browser, receiving calls or texts from anyone not in the contacts etc), and see which system lets you do that. Get a cheap phone, or an old one you’ve stopped using, and set it up safely. At that point, you’ve got the choice on when to give it to her.
I’ve given gradually more freedom to my 12yo (though still with Qustodio filtering her web access, time limits so she can’t use it at night, no social media, has to ask to download apps), but my 9yo’s old iPhone is so controlled it’s basically a brick (no web browser, no camera) and it’ll stay that way probably until she goes to secondary school.
A colleague’s 9yo was recently asked to send personal info and pics over Roblox chat, and was too terrified to tell her mum as the guy had told her that he knew where she lived and he’d kill her whole family if she told anyone - it came out by accident. That’s the sort of scenario you want to avoid, and you can set up the phone in a way that prevents that sort of thing, with a bit of Googling and some time.