My feeling is this was planned by someone who knew at least one of the girls, maybe just on a casual basis. Someone whom they would not feel threatened by if they came across him/her along a trail or in a park, in a parking lot, etc.
If their abduction happened anywheres around the lake area, I wonder what the park and/or city maintenance people wear for work clothes? We hear Elizabeth especially was a friendly and chatty child, and if the girls saw a person they recognized, dressed in clothes appropriate for that area, I think they would not have been suspicious, and readily engaged him/her in conversation and probably gone along with them without much hesitation.
I think whomever took the girls had watched them long enough, or knew the family routines enough, to have several opportunities in mind. The abductor may be someone who is so close to one of the families they know the family routines without doing much stalking or observing.
The home the girls were at that day does not sit in the middle of a group of just homes, but there are businesses around there as well. So many opportunities for someone with bad intentions to observe 2 little gals innocently riding around on bikes, like kids should be able to do anywhere.
I feel this was not done by a total stranger, it is someone at least one of the families knows, if not all the families, and it will be another case of many being shocked at who the abductor really is. I don't think that person is a sexual deviant, but more someone with not all cylinders firing, a front for someone else behind the scenes, who wanted the girls for whatever reason.