This link may help, but it does say child's weight MORE than and the first is 20 lbs which would seem that your child may be too small to take Benadryl.
You have to be very careful with all these drugs because they manufacture many different potencies -- infants, toddlers, children, adults, etc. Also, you need to keep the plastic measuring cups with the original drug they came with -- otherwise you could overdose.
I know this because my sister is a doc and there are many dangers in this area.
I am a natural health nut, so I would start with a humidifer, try oregano oil by North American Herb & Spice, try a homeopathic remedy, etc. In my opinion, these are a lot safer especially when you don't have correct dosages for drugs.