My DS1 did this when he was about 17months. It used to drive me nuts, to the point that I would only put him in all in ones and when he worked out how to undo them I resorted to putting a safety pin there.

When I finally got jack of trying to keep his nappy on he basically started going to the toilet by himself. By the time he was 19mths he was pretty much toilet trained except for poo, that is whole other story.

With DS2 I just left his nappy off when we were at home and took him to the toilet every 1/2 hour. By the end of 2 weeks he was tt day and night, only had a few accidents.

Good luck she may be ready.