Gerri, It is MRSA, I have not heard of using bactrim as the choice to treat MRSA. But that doesn't mean it is not used. If she is taking heavy dose antibiotics make sure that you check with her pedi about adding a probiotic such as culturelle while she is on the antibiotics, this will help prevent her from getting C-diff an antibiotic resistant form of diarrhea often associated with antibiotic use.
I have never heard of using benadryl to prevent MRSA from spreading (benadryl is for allergies).
Where is the MRSA? in an open cut, blood or her sputum? It makes a difference. I am guessing it is an open cut because you mention that she is using a topical antibiotic. Just be careful MRSA is very contagious, wash your hands frequently and keep the area covered. Make sure that your daughter also washes her hands frequently, especially after touching the affected area.
Here is a good link to learn more about MRSA
http://www.cdc.gov/mrsa/