Jem, if you've only had the cat a few days, I would hope you can give her a bit more of a chance. Do take the bite to your GP or walk-in centre, just to check - I've had some snaggy bites from one of my cats from time to time and it's really hurt, their teeth are sharp and can be dirty, but I've never had an infection from it. My thought is that you don't know what this cat may have experienced in the past before she went to the RSPCA, and she may still associate stroking or handling with mistreatment at the moment, and she will have to learn to trust again. You will need to tell your grandchildren to leave the cat alone for a while, until she recognises them - small people can be rough without realising it, if the cat's not happy she may go and hide while the children are about, all of mine do as soon as they hear little feet! My youngest cat only tolerates petting and grooming for a short time before he starts snapping, but we know the signs now and make him get down from our laps when he starts looking wild. I suspect it's fear and uncertainty when you stroke her which is making her lash out - she should come round in time, I certainly hope so, for both you and her. Good luck!