On the way there I explained that we would not be spending $200 on costumes, so they needed to keep the price range within reasonable limits. We also wouldn't be loading up on optional accessories, so make sure the costume they picked had what they needed. In no way whatsoever did it occur to me that we should have the "nothing too grown-up" a.k.a sexy, talk with them. I mean come on, they are still babies!
When we got there our son picked out his Stealth Ninja costume relatively quickly. He is literally covered in black from head-to-toe. Basically his eyes are the only thing you can see.