To be fair, there are different grades of missing. Circumstances are established very early by police and a level of urgency applied with regard to age and other factors such as whether they are especially vulnerable (suicidal or disabled), how out of character it is and so on.
I don’t suggest that the levels of urgency are those I personally agree with nor that they are always correct, we know for a fact that some “runaways” weee actually abducted and murdered.
But six years old is extremely young and the fact they are twins makes it more interesting. We don’t know the circumstances but perhaps there was a history regarding their safety and whereabouts that makes it more alarming.
Most child abductions are carried out by family members, stranger abduction is very rare. But stranger abduction usually leads to death within a short period of time hence the race to find the abductee.