If it's a persistant problem in the area then the Police should be doing something about it. If it is a heavily populated area there may be cctv
footage that the Police can check.
It may also be worth getting in touch with your local Councillor and see if they can do anything about it. They can help to prod the Police into action if need be, or there may be something else that they can do for example putting up temporary cctv
in problem hotspots etc.
You can find out who your local councillors are and how to contact them here...http://democracy.wirral.gov.uk/mgMemberIndex.aspx?bcr=1
I hope your daughter wasn't too badly shaken up by the incident and remembers that the vast majority of our young people are good, honest and decent individuals and that the scumbags who did this really are to be pitied...