Some families and friends we know have a picture of the beloved animal and make an 'altar' of sorts. They might place some flowers, a candle, a letter to the animal (usually to say goodbye-- to remember those good times), a little dish of kitty food, etc. in that space. You could help your daughter and family make a list of "our Sally favorites" and include anything and everything they loved about Sally or loved doing with Sally.
I don't know if this will work in accordance with your family's faith. However, with so much ambiguity surrounding the cat's disappearance, having tangible place to mourn might help, and this is not a tombstone, it's a temporary place to remember and let go. You can scale back the altar when it has served its purpose, and keep the picture. Let your daughter have the candle lit when she thinks of Sally.