Well, we have learnt by trial and error, and we are still learning!
When we first started out all we took was the GPS and some print-outs.
Now, the first thing our kids do is sort themselves out with a hefty stick - good for poking about in holes, ground testing and nettle swiping.
Now we take:
GPS
Print-outs
camera (for victory shots; and of course for any amazing views along the way, but mostly for victory shots!)
wet wipes
plasters
torch
batteries
mobile phone
water
picnic lunch if planning to be out caching all day.
swaps
pen
But we are still learning...no doubt we will discover more things we need!!