My initial thought was when Julia was at this 12 hour mediation, and IF it is true the children were left home alone, my thought was they were locked in a closet. The more I think of it now, perhaps they were both locked in the bathroom.
We don't know if Sky was potty trained yet, but let's say he was and he could use the potty as well as his sis "M". They would have access to water, however, also have access to a lot of danger by falling into an empty tub, or falling on a tiled floor or into the toilet.
If Sky was not potty trained, his diaper would be full, as well as heavy and he would want to pull it off, or it would leak out of both sides. Too much room for messing an open house, if they were left free to roam. This is why I feel they were confined and the bathroom would be the easiest to clean.