No, the two Pokémon with an IV of 31 do not have to require the power items for them to breed the descendnant.
Though in SS/HG, you can use the power items to get perfect IVs for the descendant if the mother/father has a an IV of 31, and you put the right power items on them. This would result in a really great outcome, but you are using Pokémon Black 2 :[
IV calculator: http://serebii.net/games/iv-calcdp.shtml
Guide: http://pokerealm.com/articles/iv-guide.php