You people seem to do pretty good with coming up with the whys, etc. so I'll just try another how and the blanks can be filled in with whatever reasons fit for the girls to do things (and for the perps, unless something comes to me before I finish.)
The girls were riding around in their allowed area, and met up with "someone" who told them about "something" at the lake. They rushed down there to get a look after seeing if grandma was looking for them yet. They hurried down the bike trail to the area by the double fence, dropped their bikes and took off to see "it." A minute or so later, TG rides by and sees their bikes laying on the path, swerves to go around them and continues his ride.
While in the woods/by the jeddy/up a tree/whatever "someone" comes along and invites them to a house for a quick glass of juice/water/v8/whatever before they have to go back. The girls ride over to the house with "someone" and are taken from the house by "more someones". Once everything is ready to go, 2 smaller female "someones" ride the girls' bikes over to the gate (being seen by the mystery man) and leave them, and either leave Elizabeth's purse there, or don't notice that she had dropped it there.
When that's all taken care of, they walk back to the clearing/maidens lane/wherever to meet the partner/s and the girls, get in and they all leave. All I need is names for the someones and somethings, the address of the house, and a destination for them after leaving the bikes and I'll have another full blown wrong scenario!