Well, and just in my opinion:
If all the approach is correct, I think you are going too fast and too long since you have not talked with your girl yet.
Talk with her, convince her, agree terms and conditions, and then purchase the not secure belt for those terms.
We are all girls, and we know that it is not so simple, "happy birthday, darling, I bought this skirt for you. I want you wear it"... we know it does not work with a blouse and less with a chastity belt.
And please, do not think that I am discouraging you, because I think that you have good idea, and every girl is a good candidate to your proposal, but I think you are making the things complicated and badly.
As I am more or less the age of your daughter (1 year more or less) I can asure you that pregnancy risk is small and it can be defeated with anticonceptive.
If you want protect fully to your daughter and you do not like she can have sex or touch herself, what is pretty correct in my opinion due I do not like the concept of a girl touching herself, then chastity belt is the option.
Chastity belt is huge demanding in psychological aspect, so, better talk with her before. And think that if she agrees, she become in very dependent of you. You will have to be there, even for unlock her for a shower, or sorry for unladylike, for release her if something in toilet was wrong and she need a "deeper clean".
Helen, in my home there are 3 girls wearing a belt, but I think I can not help you, since the terms of your daughter are in the air. Talk with her, build a frame of conditions, and then we can advise you to achieve the objectives on the most safe of ways.
The key in belting is the girl, not the belt.