We do have to understand how those numbers are generated: They call the sponsor the child was left with and if they don’t answer they are considered missing for this count.
That said, some sizeable portion of that actually are missing, and the left was throwing a cow over much smaller numbers when Trump was in office, which included examples (about 800?) where parents brought children into the US and left them here when they went back across the border.
Someone I know tells me the dead bodies of the little ones are often tossed in the Rio Grande or other river, once the fiends are finished.