Neighborhood Kids and the Anthony Pool
I would imagine LE has already canvassed the neighborhood looking for the "kids" who according to Cindy could have snuck into the backyard while they were at work and used the pool ergo the ladder being up.
If not, that's definitly something investigators need to do to help facilitate a resolution to the question of why the ladder was found up instead of down.
I don't know about the Anthony's neighborhood but would also imagine it would not be too difficult to find these kids based on interviewing neighbors for ideas/suggestions on which one might have been involved or perhaps have a history of doing such things.
Given LE is not likely to charge these kids with any crime (wouldn't the Anthony's have to press tresspassing charges??) I would expect if this is what happened it should not be too difficult for LE to find the kids and interview them with their parents knowledge and consent.