You don't need to take her mythic belief in Santa away now to protect her from others telling her first. Your responsibility in this matter is to abide with her when her fantasies and myths grudgingly give way to more mature, confusing realities.
Through natural child development, most kids will surrender the Santa myth by age 9 or 10. Let her continue her enchantment. It's only this pure once. She'll let you know when she needs your help to navigate the waters from childhood fantasies to adult realities.