Trying to convince her of that presupposes that a sex toy IS the best thing for her.
1. It may or many not be true. If the issue is her current lack of sexual satisfaction, talk it out first, work on skills/techniques, etc. first.
2. "Sex toy" is a pretty broad category. Choose the wrong one(s) and that might be the end of it.
But if she's resistant to the idea, for heaven's sake, DON'T make it a Christmas gift. Get her something you know she would like, and get a sex toy some other time.
Get her a Sybian or one of its various knock-offs. Might be acceptable for Christmas, birthday and anniversary all rolled into one, but she'll never want to leave the house.