Dear Martin,
I have tried a debug activity.
myTableParent = element.args().record();
myTableChild.MyID = myTableParent.MyID;
//mytableparent return recid=0
But, I have changed myTableChild.MyID = parentTable.MyID;
into myTableChild_MyID.text(myTableParent.MyID);
now it's working, it return the value i inputted, I don't really know why myTableChild.MyID = myTableParent.MyID; not working and myTableChild_MyID.text(myTableParent.MyID); working.