>I'd be interested in your test results. Now that I think about it a bit more, the FOR condition should be Rushmore optimizable so there may not be a big difference.

TestSpeed.prg

local lnMax, lnMin, lnI, lnK, lnParent
if not file('Parents.dbf')
	create table parents (ParentID I)
	index on ParentID tag ParentID
	create table Children (ParentID I, ChildID I)
	index on ParentID tag ParentID
	rand(-1)

	lnMax=1
	lnMin = 10000
** Create test tables
	for ln=1 to 500
		lnParent = int((m.lnMax - m.lnMin + 1) * rand( ) + m.lnMin)
		if not indexseek(m.lnParent,.f.,'Parents','ParentID')
			insert into parents ;
				values (m.lnParent)
			for lnK=1 to 1000
				insert into Children ;
					values (m.lnParent,m.lnK)
			next
		endif
	next
endif
local lnSec, lnRecno, lnChildren
select Children
set order to ParentID
select parents
set relation to ParentID into Children
lnSec=seconds()
scan && Parent
	lnParent = ParentID
	select Children
	lnRecno=recno()
	count for ParentID = m.lnParent to lnChildren
	goto m.lnRecno && Restore record pointer
endscan
?seconds()- m.lnSec
go top && in parents
lnSec=seconds()
scan && Parent
	lnParent = ParentID
	select Children
	lnRecno=recno()
	seek m.lnParent
	count while ParentID = m.lnParent to lnChildren
	goto m.lnRecno && Restore record pointer
endscan
?seconds()- m.lnSec

Results:
First run: 1.106 - 1.854
Second run: 0.448 - 1.868
Third run: 0.490 - 1.846

E.g. count while seems to be much slower.

May be it's not a really good test, what do you think?