I think you make a good point. There was one summer when my son fell in with some unsupervised children. He was not honest with me about what he was doing and when he was late, I didn't even know where to begin looking for him. Looking at the location of the Collin's home, it would make more sense to look along the river, which is only a couple of blocks from the house. The decision to look at the lake suggests that someone knew that it was a likely spot.
It could be as simple as one of the relatives knowing that the girls were sneaking off to the lake without Grandma knowing what they were up to, and keeping the secret. Some adults want to be a child's friend, rather than an authoritarian figure, so they keep children's secrets. The person that first suggested the lake is probably the relative that kept the secret from Grandma and the parents. That doesn't mean that the relative was involved with the abduction, only that the relative did not enforce the rules and, as a result, the children were vulnerable to abduction.