It doesn't sound as if they were in car seats. One child was on the front floorboard and the other was lying in the backseat.
I think it is very possible they could have been hiding from their mom, as TGIR suggested above. Maybe they were playing, maybe they were told to stay there? I don't know, but something is wrong. The doors were unlocked. Also it says Mom discovered them missing about 1:00 p.m. and called LE at 2:10 p.m. That's an hour and 10 minutes. I'm not sure I would have waited that long. Originally it was said that it was 40 minutes before mom called LE and while I thought this was reasonable, to me, it was on the Long end of reasonable. But if others were helping her search, who knows. But an hour? Would you wait an hour if you couldn't find your 4 and 3 year olds?
Also says mom looked in the car at the curb but didn't check the car in the driveway. I wonder why? Maybe it was someone else's car and she didn't think the kids would get in there? I don't know, but this is distressing.
Salem