You are assuming in this speculation that the parents are together or are co-parenting normally.
If however that is not the case, I can easily see why one parent believes the DC is with the other, and does not report them missing until after the set return time (and a bit of time trying to track them down).
But we simply do not know what the family background is here.
What we do know is that a 6 yo is missing in SE London, and she has not been seen since the CCTV images yesterday lunchtime.
I really hope that people in the area might remember seeing her, and give the police new leads.