Huh... this sparked a thought I hadn't had before. Although I think I just talked myself out of this idea, I'll post it just in case it helps us think of anything.
Would the girls have willingly gotten into a vehicle along with their bikes, so they could be driven to the lake for their bike ride? So they'd have a little longer to ride? It'd have to be someone they knew well in that case. The reason I talked myself out of it is it seems like it'd take longer to load and unload the bikes than to just race down to the lake.
I think anything reasonable is possible at this point and this seems reasonable enough.
The thing that keep tripping me up is: when I was young I would venture farther than I probably should have, but never when someone at home was expecting me at a certain time. Unless they wanted to get in trouble, I sort of don't think they headed to the lake that day, even willingly by car.
That's why I like my theory of an abduction on the nature trail, but nearer to the house. It makes some stuff fit for me that didn't when I was thinking of them going to the lake on their own.
-it fits with them being pretty good kids who listened to grandma.
-it helps with the timeline because the bikes would be traveling by car.
-it gives a reason for why some one would dump the bikes at the lake (the girls were already on the trail, just at the other end of it)
On another topic (and completely opposite from what I just said, haha), I'm not exactly sure what the deal is about a paddleboater, but I haven't yet seen the possibility brought forth that the girls were the ones boating. Since we're here to throw out ideas I'll throw this one out there. Maybe the girls had seen the boats there before or heard about other kids sneaking a ride and decided to blow off curfew and take a little joyride. Maybe someone saw them do that and used their naughtiness to his advantage ie "Why are you using my boat?
(a lie) Girls come with me. We're going to call your parents". (I heard of a rapist using this kind of tactic before. He would pretend to be an undercover security guard and "catch" teen girls shoplifting, take them to the store bathroom and "strip search" them.)