I have a hard time believing that, out of 4 people, nobody would have noticed she was missing until morning.
I actually had a similar incident happen to me. I had 5 children and my boyfriend had 3. When going from my house to his we would load up the kids in the van. One day we did this and I cooked dinner at his house. Once dinner was ready I called for everyone to come and eat. My youngest wasn't there. I raced over to my house and there she was sitting on the steps. No one mentioned she wasn't there. You talk about feeling horrible! I thank God nothing happened!
Maybe the senario of forgetting her happened. But I think they would have raced to the last place they saw her first. Maybe they did. We don't know if someone noticed Isabelle was missing after they got home. It is possible they did go back to look for her at the park but couldn't find her. The video could show if their car left in the middle of the night. The park could also have cameras that would show the activity.
Praying for Isabelles safe return.