11mo f c rash on upper body and face. Possible reaction from cephalexin. Taking Benadryl (diphenhydramine) q 4-6hrs prn. She's still pretty uncomfortable. Suggestions?

It could be due... ...To the antibiotic, or it could be secondary to the infection that the antibiotic is being used to treat. Or it could be something else entirely, such as a viral illness. Benadryl (diphenhydramine) is not going to do anything except make her drowsy. If she's uncomfortable, you should let your pediatrician or a dermatologist examine her at your earliest convenience.
Cortisone. You can use Hydrocortisone cream safely for 7-10 days and this should help. Oral steroids are very helpful for severe reactions to medications. You would need to see a physician to have these perscribed.