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How old are the kittens?
Thank you for the extra information.
You are absolutely right about the 1 mg per pound of body weight. However, you never want to give a kitten under 6 months old any OTC medications. When it comes to Benadryl, you shouldn't give any cat at any age Benadryl. Cats have been know to have adverse reactions to benadryl.
Benadryl is best saved for dogs. Dogs do better with such medications.
For cats you want to use Chlor-trimeton, if you are going to give an antihistamine. But again, not under 6 months old.
I am sure given the tiny amount and you've only done it once there will no problem from the benadryl, but don't give them any more.
Keep up the prescriptions from your vet. Use steam from a shower to help break up congestion and a vaporizer where they sleep.
You can use Afrin nasal spray to help the congestion in the nostrils too. Even squirting plain saline up the nose will help.
If they are not eating, just get some KMR kitten milk replacer and feed them with a medicine dropper. There is a feeding chart on the back of the KMR packaging. You can find it at Pet Smart and like pet stores.