The family says they have never biked that far before.
Even if they had been there before, how was he to know that was their destination this time? They could just have easily have been going to a friends house, yet he still took the risk of driving ahead of them and waiting at the lake on the offchance that was their destination that day under the "stalker" theory.
He took a very big risk for a very uncertain result if he didn't already know they were going to be at the lake, I believe.
I honestly believe the girls died on 13 July, the same day they were taken.
A sexual predator with 2 little girls would most likely keep them for longer than one afternoon, and have somewhere to take them and keep them a bit less public than 7 bridges. Even rock spiders have friends who own houses and like to share.
By the way I found a youtube clip of the area they were found, it is very gloomy and out of the way. It is also on private land, has a house within easy walking distance, and is completely accessible by ordinary car because it is kept mowed right up to where the woods begin so someone could easily have driven right up then walked the girls in or possibly even carried them although this seems unlikely as the undergrowth in the wooded area was very high at the time.