Benadryl is for allergic reactions to a strong allergin. Hives, swelling, etc. It should not be used for just a runny nose and stuff. It dries their sinus' out, they get super glue snot stuck in their sinus' and throat. It is not the best thing for them to take. It may stop the runny nose but think about where it is storing that snot and what is happening to it. It will sit in the sinus cavity and start building an infection. It needs to get out, it's job is to wash away irritants. The super glue goo will find a way out, it may try the ear canal, get stuck and cause an ear infection, may try the sinus drainage route, get thick, green, and make a powerful headache. It may also try to get out through the eyes and cause mucky eyes and redness that looks like pink eye but is really only sinus drainage that is trying to get out.
Take the kiddo to and ORL/ENT doc and get the recommendations from the horses mouth.