The problem I have with believing that two different sets of children left their bikes on the trail that particular afternoon is that local people have said that the trail is not used that often and that very few people actually walk/cycle all around the trail. We also know, so far, that only one "runner" is confirmed to have seen the girls bikes on the trail between noon and about 4. Also, if two other children were playing in the area at the same time that the girls bikes were at the trail, why didn't they see the girls? It's possible that two sets of children left two sets of bikes on the trail, but I think it's unlikely.
Maybe the first bikes (seen by the runner) belonged to someone who was waiting to meet the girls, maybe pre-arranged, someone who had befriended the girls for unknown reason. Maybe they were in or near the lake, fishing, throwing rocks, or in the woods while they waited for the girls. The girls showed up, parked their bikes (in the location found by LE) walked to the other side of the fence, set the purse down and forgot it, and left willingly with the "friends". Maybe the "friends" rode their bikes into the woods or further along the trail with the girls either walking or hitching a ride on the bike. Maybe they went to a nearby house or (if "friends" were older) to a car/van/truck. From that point...something went wrong because the girls never returned to their bikes!
IMOO...I think the girls went to the lake to meet someone and something went terribly wrong! I don't think the bikes were staged. I think there were 2 sets of bikes, which is why there is little info given about their descriptions, and why there is such conflict with the timeline. With this theory, the girls would have been on camera, the runner would have seen 2 bikes (not the girl's bikes), and Mr C would have seen the girls.