As mom to a "rambunctious" little boy, I don't find it impossible to believe that the kids wandered away. But I cannot understand why, especially in an area full of environmental hazards, they would not call law enforcment immediately.
Our 4-year-old slipped out of the house one time while I was getting laundry out of the garage and his dad was in the bathroom. As soon as I walked back into the family room and saw that he was no longer painting with his sisters, we had an all-hands-on-deck alert. We spent at most 3-5 minutes doing a cursory check of bedrooms, closets, fenced back yard, unfenced front yard, etc., then called 911 and started beating on neighbors' doors to ask for help.
We knew that several neighbors had swimming pools, that there was a busy road just a couple of blocks away, and that time was NOT on our side. Our only concern in that moment was finding him before tragedy struck, and we wanted as many people as possible helping us do that.
Thankfully, we found our little one safe (albeit terrified because he'd crawled into our next-door neighbor's new car and gotten himself trapped by their child safety locks) before police ever arrived. Then, we installed loud door alarms and security cameras. ;-)