You are such a good grandparent to care so much about your grandkitty!
There are some medications that can be given to cats to help with anxiety. I do not use valium in this manner as there have been reports of cats on long-term use going into liver failure. Cats are very sensitive to medications due to a difference in their liver metabolism so never, ever give your cat a human medication without a prescription from her veterinarian. They can be sensitive to herbal remedies as well.
You can try some things to help her for now. If she does not routinely get canned food, start giving her a little. If she likes it, it can help her feel less nervous and the extra fatty acids in it can help her skin. Make sure she has places to hide like a box or bag or closet. High places for her to sit and look down on everything help also. Try spending some time with her grooming or petting her or try to interest her in playing with toys. Try to keep things on her usual schedule, feed her when your daughter usually does, etc.
You may be able to find a calming remedy at the pet store. Sometimes these work, a lot of times they don't. You may also be able to find feliway at the pet store. This is a pheromone product that can calm cats.
When your daughter comes back, have her talk to her veterinarian about an anti-anxiety medication or have her bring her to a veterinary behavioralist. She should be able to make her better.
I would not suggest using any human medications in cats (or dogs) unless specifically directed by a DVM.
I can't say for sure this is medical or behavioral, but sounds like it has an anxiety component because of the increase when you mention it gets worse when your daughter leaves.
However, medicated shampoos helping seems more of a derm issue.
Valium can be used in cats for anxiety and especially if this is a temporary environment it may be worth exploring with your veterinarian. I've asked our behaviorist to chime in here, and perhaps Dr. Karnia will also give her opinions.
Your daughter must be comforted knowing she can leave her cats with someone who is so attuned to their welfare. Congrats on being such a good "grandma" ( years ago I gave my father a bumper sticker that said "Ask About My Grand-Dogs!").
From a behavior perspective, I'd suggest having your daughter bring along some old items of clothing that have her scent on them the next time the cats stay with you, along with a favorite cat bed or window perch. Lots of hiding places as Dr. Karnia mentioned - even simple things like paper bags or small cardboard boxes lying around. There are great kitty tunnels and cubes made from rip-stop nylon at most pet stores as well if you want to get fancier.
The Feliway plug in may help, but in my experience clients have given quite mixed reviews on that - so I wouldn't hold my breath! Does the cat have any experience with catnip? Some cats become really mellow after playing with a catnip toy, but a few can become quite grumpy, but that's another thing to consider.
If the cat likes to be held and petted you could certainly consider spending more time with her doing that. Or she may just like being by herself in a quiet room, or even a high perch such as on top of the fridge - so if she likes being high try and give her access to safe, high places.
Don't know your situation but would it be practical next time to have the cats stay home and you either stay at your daughter's house or go over couple times a day to feed and check on them? Some cats are much more upset by new places than anything else and they might be less stressed staying in a familiar environment.
Veterinary behaviorists should weigh in on the medication issue, but just to relate a personal experience, - years ago we were moving cross country with 2 elderly cats and our veterinarian suggested benedryl to keep the cats calm - it worked great. They slept a lot but at least they weren't yowling in the car as usual.
If you want to try a naturopathic or homeopathic remedy, in my experience the ones that are available only through veterinarians who specialize in alternative medicine work better than any herbal remedy you can get over the counter. As Dr. Jim said don't give her any medication or remedy without your regular vet's input.