Like what was said earlier, I'd be really careful when wearing fragrances around the con. People tend to be allergic to that stuff. I should know, one of my cosplay buddies has all sorts of allergies. I had to step out of the hotel room to spray on my perfume because she's so allergic to fragrances. My suggestion is to look for perfume oils or scented lotions/body butter since they're less harsh. In addition, a cheaper alternative is to look for travel size or sample size versions of the scents you want to wear. That way you want have to buy a full bottle.
Back on subject to your characters!
Boy Ciel to me doesn't seem like someone who has a scent. If anything, I think he would just smell clean like most male characters. Victoria's Secret has a brand of scents from their "PINK" line. One of them is called "Fresh and Clean", which smells like clean laundry.
For Girl Ciel, I'm not too familiar with floral scents. The only one I own is called Kimono Rose from Thymes, which is a light floral scent but I love it all the same. Unfortunately, it's impossible to find in stores. I had to get mine online.
Costumes for 2013:
Seras Victoria-Blood version