If the chick is still very small it might help to cut the tape in half length wise. "Wrap" it around one leg and then another - it's better to put some tape on the sticky part lest the baby get stuck or get bedding stuck on it.
Try medical tape instead of string, it'll be a lot harder to come off. Gradually bring the legs closer together over the course of a week or so, since she's a bit older. Make sure he substrate she's on isn't slippery (like newspaper-way too slippery). Hope she gets well soon!!