Actually, experts say they should have reported them missing sooner. Studies have shown that most abducted kids who are murdered too are killed within the first 3 hours (with the highest number within the first hour.) I think they called as soon as they did because the girls didn't normally stay gone that long, and normally were close enough to hear when they were called (and returned home when they were.)
They had been searching a small town for 2+ hours before contacting the police, and these girls are pretty young. It would have taken me a lot less time to call if one of my sons had disappeared when he was 8, but I guess that's just me. I was lost once when I was 10 (I grew up in Minneapolis) with a friend who was the same age. We were gone for at least 3-4 hours, but our mothers never knew we were missing because they assumed we were at the other one's house. It scared me enough that when my son was that age, almost 25 years later, it still bothered me. He hated that he had to check in at least once an hour, always tell me where he was going, and let me know if he decided to go somewhere else before check-in time.
He was allowed to go about the same distance as these girls; down to the end of the street (3 blocks away), as far as the school in the other direction (2 blocks away), to the church on Wednesday afternoons for their youth group (3 blocks away), to a park 1/2 block away or the parking lot of a business across the street. He'd often ride with friends around those areas for hours, making sure to stop by often so I knew he was still around. At 12 he was allowed to go on the local "bike trail" but it was one house away from ours and only a few blocks long. They just liked it because they could ride faster without worrying about cars.