int virtualChildBase

The virtual index of the first child.

When asking the delegate for the position of each child, the grid will add the virtual child i to the indices of its children.

Source 

int get virtualChildBase => _virtualChildBase;
void virtualChildBase=(int value)

Source 

set virtualChildBase(int value) {
  assert(value != null);
  if (_virtualChildBase == value)
    return;
  _virtualChildBase = value;
  markNeedsLayout();
}