HELP Flat IRON..SINGLE Father with girls

Hi I'm a single father of 3 beautiful lil girls..I'm learning to flat iron my 8yrld's hair and I can't seem to get the "Friz"out where here hair will lie flat and  like her 14yrld sisters?Both are Bi-racial childlren.But my 8yrld has typically good non-course hair.My 14yld old has mostly course hair and I can get her's to lie flat and look like it was professionaly done.Neither has perms nor needs them.Do I need to by different poducts to flat iron.Different methods?I wash both and comb,and treat both with a "TRESemme thermal or John Fieda's FRIZZ-EASE..After doing my 8yr old some spots look great but it mostly looks poofed and frizzy...I do realize this a very general post but any help will be greatly appreciated....

Edited by ezd20 - July 14 2011 at 7:21pm

Try using Pro Naturals Moroccan Oil Hair Treatment with Heat Protector which reduces the frizz and prevents heat damage from the straightener, leaves the hair shiny, oft and manageable.