I have done it though I'm in no way an expert or convert for that matter. I think most people do locally or as an activity on camp or holiday. There are some areas where there are lots of cashes, some more sparse. For instance the Region did a days training around York City Walls once as there are plenty in the vicinity.
If your girls are novices it may be worth setting up your own 'mini-trail' for them to follow...for more ideas speak to your County Walking advisor, they may have some ideas or equipment for you to use.
Have you had a look on geocaching.com? You can set up an account for free which allows you to search by location/ postcode and view the results as a map, so it's easy to see where the geocaches near you are, plus it also has info for each cache on whether it is easy or difficult to find and what the terrain is like in the area. I'm in another area of the country to you, but I can see for example there are currently two or three within walking distance of our meeting place, or about a dozen at a local park if we got parents to drop off/ pick up there instead.
Some campsites have their own geocaching activity set up on site too
NEE did a training event in Sheffield a few years back for the leaders and it was great fun. I am already a member of the Geocaching website. I was thinking maybe a few of us leaders should meet up for another Geocaching session in the near future. What does everybody think?