My youngest could open up the front doors on our mini van when he was 20 months old or so. (This is the wild child I have tons of stories about.) He was even determined enough to figure out what key went into the ignition. How do I know? My husband was working under the hood of the van with several people around and baby boy opened the door and got in the driver's seat. No big deal as he was contained and easily seen. My husband took my keys out and let him play drive. What he didn't know is I had the spare keys in the van and that little stinker found the right key, put it in the ignition and turned it to accessory and put it in reverse. HORRIFYING!! Here is what ended up happening.
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We sold the van, got one with shift lock and added never letting a child play in a vehicle no matter what to our ever growing list of things we needed to do to keep this child alive. So, I can understand how these 2 could get into the vehicle on their own. The story may not turn out to be what happened, but I do wholeheartedly believe it is plausible and I am assuming mom spent the 15 minutes looking for them. Yes, I would have called LE before then, but it sounds like the children were mimicking what they see by getting in the vehicle with the keys and phone. (I want this to be what happened because my heart cannot take another parent intentionally harming their children.)