Some vets recommend using a dose of benadryl to fight the allergens. You should give your vet a call and ask about it. Cats have very difficult central nervous systems so I do not recommend this unless you have your vet's approval. If he says go for it, the normal dosage is 1 mg per lb of body weight given every 8-12 hours.
If there is drainage from the nose, you can use pediatric (not adult) nasal saline drops to help clear the mucus.
I would keep a close eye on this and see the vet if it does not start to improve. Your cats might be suffering from an upper respiratory infection (hence the sneezing) so you want to stay on top of it and see the vet if they stop eating or the discharge from the eyes/nose becomes yellow/green.
Let me know if you still have questions.
