It's been many years since I had to study this stuff for the Bar Exam so I might be a little off on my analysis. From what I remember your son's friend is an "invitee" or "licensee." You have a duty to 1) make a reasonable effort to learn/discover any hidden dangers or hazards on your property and 2) warn him of any known hazards including "apparent" hazards such as a lake or ditch, pole, etc. (Yeah it may mean that you should spend a few minutes pointing to the trees, fences, rocks, ditches, structures, ponds, tanks, etc. on your property and telling the kid to not run into these.). Your son's friend has his own duty to then avoid such hazards and a general duty to conduct himself using the usual care and not recklessly. To the extent that he might be shown that he was injured due to his own contributory negligence any damages and liability might be reduced.
Question: will the cycles/vehicles your son's friend is using be lent by you or are they his own? If they are yours or your son's then you have some duties about inspecting and making sure these are in working order.
In any case, obtaining a release from liability (SIGNED BY BOTH THE CHILD AND HIS PARENTS IF HE IS UNDER 18) is a very good idea and may be useful in every situation except where there was some hidden or apparent danger that you did not disclose and that you should have been aware of. In other words the kid/parents can only release liability from the potential dangers they are aware of. The liability form should be specific and probably reference the activities, risks from such activities and approved times that these may be engaged in. Wearing a helmet or other safety equipment should be mandatory and conditional to the granting of the license.
Finally, make sure to check with your homeowner's insurance company to make sure that the activity will be covered and will not in any way be a breach of your policy.
I know all of this is crazy and it is too bad that kids can't just have fun like we used to in the old days but one has to be careful in our litigious society.
-Eduardo