I think it is possible to let your children believe in Santa even though other people tell them he doesn't exist.
Much in the same way that small children will still believe in the monster under their bed even though you tell them monsters don't exist.
For example, you could ask a neighbor or somebody else your child isn't too familiar with to knock at your window dressed up at Santa, so the little one can see him. Seeing something can be very convincing to a child, and leave a stronger impression than any spoken words.
The question is, would it cause problems in your family if your daughter would openly disagree with grandma on this topic, or would it just put a stop to the discussion?