The ParentObjectNumber corresponds to the ObjectNumber (ordinarily the second column of the spreadsheet/database) of the object in the previous frame that was linked to the object in the current frame.
The “TrackObjects_Label”, as you’ve noticed, propagates to both child cells after splitting. However, both “halves” will have a unique ObjectNumber at every frame- aka at Frame 25, the top half of your blue cell likely has ObjectNumber=1 and Label=2, the single green cell (which has not yet been split) likely has ObjectNumber=2 and Label=1, and the bottom of half of the blue cell likely has ObjectNumber=3 and Label=2.
Consequently, at Frame 26, the top half of your blue cell likely has ObjectNumber=1 and ParentObjectNumber=1 as well as Label=2, the top half of your green cell (which is now splitting) likely has ObjectNumber=2, Label=1, and ParentObjectNumber=2, the bottom of half of your blue cell has ObjectNumber=3, Label=2, and ParentObjectNumber=3, and the bottom half of your green cell has ObjectNumber=4, Label=1, and ParentObjectNumber=2. The parent number tells you how to
You can therefore use the ObjectNumber and ParentObjectNumber to trace ChildA and ChildB - since you’ll always have a unique object number at every frame, you can find how to connect them to the frame before using that metric.
Does that help at all?