My husband was asked by our neighbors to play Santa, for several years running, for their little boy. They rented a suit and arranged formal visits. It was great fun while it lasted. They finally decided (when the child was about 8) to let him in on the secret. We handled it as if he were now old enough to be initiated into the wisdom of the elders, and he was assigned to be one of Santa's helpers, and to pass the traditions of kindness, love, and generosity on to the world's future children (though he is an only). There was no puzzlement, no sense of betrayal, because of the passing of the torch.
You could have a "tooth-fairy initiation" ceremony, or even an all-tradition initiation, to let your daughter know you trust her with this grown-up information. Mark it with a document, or a locket, or any appropriate symbol.
Assure her, if she's worried, that because parents are automatically nominated to carry on the tradition, you will continue to leave her money in exchange for her teeth if she'll feel that as a loss. It doesn't need to be handled in a traumatizing way for her; after all, you and daddy have been the ones fooling her up till now.