Randomish two in the morning thoughts...
Why THERE? I think the location the bikes were discovered is important.
This location is apparently the only place along the path that one would expect to be unobserved. It is apparently very small, perhaps only a couple dozen feet long. So why were they left there?
The girls did not HIDE them, the bikes were left (reportedly) in plain sight and perhaps actually blocking the path.
This (random late night thoughts again) leads me to conclude that the girls encountered someone at that location. However, the odds of a child EVER encountering a random maniac (for lack of a better word) are vanishingly small. And the odds that they would just happen to stumble across one THERE, at this location, by random chance, stretch believability beyond breaking. It is beyond far-fetched.
Which leads me, at this hour, to conclude that the girls had visited that lake previously, perhaps several times, and that over the course of these visits they made a special friend who wasn't really their friend at all. I suspect, random thoughts again, that this friend anticipated their coming and waited for them there. This was someone they knew and were comfortable with, and when they found him on the path they were not alarmed at all.
Just my opinion and all that.