I have a 12 lb Toy Fox Terrier and I give him adult Benadryl. Usually one capsule in the morning and one in the evening. The children's Benadryl didn't even touch the allergy and our vet recommended the regular dose instead. It works much better.

Don't worry about the vet giving a shot of cortisone. If you don't want it, then don't allow them to do it. She's your dog, so it's your call as to what treatment you want to give her. Don't not take Lexie to the vet because of them wanting to give her a shot. She can get a staph infection and she'll need antibiotics for that.