I don't remember Noah's exact birth date, but he was getting close to 6. He was in kindergarten. You still keep an eye on kids that age, but it's not unreasonable to expect them to watch tv for a bit while you do something else. My guess is that she didn't intend to sleep more than about 30 minutes, if that. She was probably expecting the baby to wake up at any moment.
I am all about parental accountability, and have some major issues with kids being killed due to their parents' negligence (Jenise Wright, for example), but I also think there are some things that are reasonable. If she'd left him outside to play and then took a nap, that would be inexcusable. But at that age, inside the house, with rules explained and assuming he was known to usually follow them, I don't think she was being a bad mother. I'm also assuming it was a rare occurrence. I napped when my kids were that age, but I did make them stay in the same room with me because I never slept so deeply that I couldn't hear the bedroom door open, so I felt like there was still some level of supervision.
Of course, this is all assuming the truth was told and she was actually taking a nap.