Hi nic31600,
Don't worry! If you speak to her Only in English, she will learn it by 100% sure. One of my friends lives in US and she's Persian. She speaks with her children Farsi only at home! And she taught them writing in farsi by her own.
Now both her children can speak and write in Farsi. (21 yo and 17 yo)
Although her son refused speaking Farsi at first, but she told me that she didn't answer him and asked him to repeated in Farsi then she answered him.
You can choose to show her only English cartoons, movies or music.
Good luck!