Having an earlier time line could bring up the possibility of different delivery people being seen or traveling through the area or witnessing something they didn't think important because of the time frame.
I have never thought that Wylma or either of the parents have been involved in the abduction and murder of the girls. I've only questioned their time line. I have clocks in almost every room, but some people don't have any clocks except by their bed and only use their cell phone or go by tv shows starting or ending to check the time.
If the girls were seen near the lake, could they have entered or left the park through Maiden Lane to make their loop to go back home?
It may have been a delivery person, but it could also have been someone that stopped at the Auction shop, or who wanted to cut down Brovan after leaving the mall parking lot ... to head South on River ... maybe that's why the children were last seen heading West on Brovan ... maybe they then went South on River to Meyers Lake Park ... following someone that they thought was going to give them a present/surprise (Disneyland, pony ride, party, boat ride).