The appropriate answer would be how safe is it around your local area? How safe is the U.K. in general? There are drinks and drugs on every street corner!!!
The festival environment is no different. The majority of people are on one sort of drug or another (tobacco, alcohol...) We would be telling lies if we said people were not drunk. However, the majority are friendly, welcoming and nice and if you use common sense, there willbe no issues.
Your 14 year old daughter MUST stick with the 18 year old friend at all times though.