Hamsters do not need bathing full stop unless as has been said, a vet prescribes an anti-fungal or mite shampoo or treatment. You could be making their skin worse by doing so especially if you add anything to the water. Sand baths can be offered as the others have said as a natural way for them to roll and sit in.
Chinchilla sand (not the dust) is sold in all pet shops and is easily sourced. Just place a few inches of sand into a low sided ceramic dish and let the hamsters discover it for themselves. Once your hamster has flakey skin or a dermatitis, it could become worse and painful for the hamster, so from today, I would look at offering them all a sand bath daily and just let them groom themselves.
Who told you hamsters needed bathing?