Never attempt to persuade a girl to abandon her dildos. Encouragement is the key. Buy her dildos, many of them, of all sizes and shapes. Long fat ones she'll struggle with, little fun ones she'll hammer away at, and all things between.
Fingers are nice, obviously. But to discourage a girl from enjoying herself with a toy is like saying its shameful for her to experiment at her terms. They are her terms. Not yours.
I was in a smut shop one time, looking at vids for sale and browsing around, and in those stores theres always a couple weirdos lurking around. They always have a few big walls of sex toys, rubber pussies and dildos and all sorts of shit. This chick comes in alone, she was in her late teens and quite obviously curious about buying a dildo. Well, once you've steeled yourself into walking into a store that is taboo, and you get inside, its never what you imagined when you're outside in your car, about to take the plunge.
She was probably never in a sex shop before. She walks in and sees the wall of toys, heads turn, she walks to the wall and starts browsing. They have everything. These assholes over in the corner are giggling and making comments about a girl buying a dildo and she overhears it. She pretends she didn't, but I can tell she's become very aware of herself standing in front of a wall of dildos and after a few minutes of trying to keep her game face, she bolted.
She left with nothing. I was really fuckin pissed. She woulda eventually picked something out, took it home and experimented. It took a lot of balls for that girl to walk alone into that store and they fucking ruined it on her.
Don't be a ruiner.