Yes, you do have the right to enter the house at any time without permission. Your granddaughter is not a tenant and you are not a landlord. Rather, you are merely a homeowner, and your granddaughter is a house guest. Accordingly, you may come and go as you please.
If you create a landlord tenant relationship, then that certainly would give your granddaughter more rights, and you would have fewer rights. If you leave things the way they are, then you have more rights and she has fewer rights. For example, if she were to do something that you do not like in the house, then you have the right to kick her out. You would not have to evict her first. On the other hand, if she were a tenant and did something that you did not like, there is a good chance that you would have no recourse at all, or at the least you would have to evict her in court.
If I were in your shoes, I'd leave things the way they are.
Does that answer your question? Please let me know if you need clarification, as I am happy to continue helping you until you are satisfied.