Having looked at map of where the bikes were found, and the fact that child's purse was on lake side of fence, two observations:
a) If you were riding your bike from downtown, they went a very long way around the lake to get to the lake edge. Their bikes were on the far SouthEast corner of the lake, the only part of the lake where a road is nearby and shaded by trees. On such a hot day, they would have had to travel well over an extra half mile to get to the far corner of the lake.
b) there is a fence between the trail and the lake, which looks like a six or even eight-foot chain link fence. The girls purse was found on the 'lake side' of the fence. (this fence is probably there to help keep deer off the highway.
These two facts seem to indicate that the bikes and purse were dumped there. The end of the tall fence is a couple of hundred yards past where the bikes were found, and there is no way the girls got over it. It does not seem logical to leave your bikes on the trail, backtrack a long way to go around the fence, and then drop your purse on exactly the other side from where you left your bikes.
Somebody dropped their bikes there and threw the purse over the fence.
Goto Bing maps and pull up the birds-eye view of Evansdale IA
http://wcfcourier.com/news/evansdale_search/
this shows where bikes were found