My DD (11) has worn glasses since she was 2. 3d glasses both at Disney and at the movies fit over them. If you are at a movie that has kid and adult sizes, you probably want the adult size- that is what DD always wanted over hers.
Also I highly recommend getting some ear grips/hooks. You can’t see them, and they will keep her glasses from slipping down her nose and her looking over them instead of through them, and also will prevent them from falling off -not only on WDW rides, but during regular daily activities. My DD has kind of a ski slope nose and those were a lifesaver– I don’t know how she would have kept her glasses on her face without them. She has had those on her glasses for years. She has never liked the look of straps- and the grip/hook/keepons can’t be seen at all with her long hair. I think you can probably get them on Amazon- I’m sure other places as well. You want the softer ones– silicone maybe? Also, while she is getting used to glasses in general and when she gets new pairs- both with and without the ear pieces, but especially without– keep an eye on the backs of her ears. We felt awful because we did not know to do that and because her glasses were slipping and she has really sensitive skin- they made blisters and deep red marks all down the backs of her ears. If hers look like they are starting to do that- they likely need to be adjusted. The ear grips/hooks/keepons help keep that from happening too.
We also have a rule that if she takes her glasses off at WDW (or really anywhere that we are with her)- she has to hand them to one of us. No- setting them down places.