This makes me really cross. Your daughter already has a dog, and then gets another without really researching the situation. It worries me that her ignorance of dog behaviour is going to cause her problems in the future with the staff and it is going to end up being palmed off on someone else. It was irresponsible of her - but you did step in to help.
Cavalier king charles spaniels are lovely dogs, that tend not to be seen as a family dog, for a reason. Not because they are vicious, but because they don't like being pulled around. So with older children they are fine, but they really just want a quiet life. It sounds like this particular dog has had a bit of a rough ride of things. It is a bit much really to expect her to suddenly accept being mauled by a wee lad. Which to be fair, is what children do with dogs.
I have a three year old dd and a year old jack russel type dog. She gives him such a hard time, and i do worry occasionally - but he keeps coming back for more , because he is a terrier and its all a big game to him. He has growled at DD a couple of times but this has been the play growling that dogs do when they play tug, so not a warning growl. A warning growl is one step away from a bite, it is a dog that is saying, look, leave me alone, i don't want to bite you but if you don't fuck off and leave me alone i WILL bite you.
With a dog this size, chances are it will be a bite to the face as it will most likely be if he hurts her or is smotering her. So, please don't be complacent, she might not be a rotweiller but she can still scar your little lads face for life.
In all seriousness, you really have to consider if this is fair to this dog. She has obviously not be raised with children so wont be accustomed to all of this pulling around. I feel quite sorry for her tbh. Contact your local dogs trust or cavalier rescue centre or RSPCA Before she snaps at your son, because once she has bitten, she will be classed as a biter and will be difficult to rehome, through no fault of her own Poor dog, she would be the ideal companion for a retired person who would be able to give her the peace and quiet she deserves.
I think it was lovely that you tried to help out, but please don't give the dog back to your daughter. Also, advise your daughter very strongly to get some proper training in place for her staffy, these are strong little dogs that really WONT take any shit from anyone.