Found: Sci-Fi, YA, Kids/teens can see creatures adults cant

I read this book back in 2002-2005
Kids could see these creatures floating around adults. The creatures were feeding or attaching themselves to them to either suck thier life force or youth? I think they were floating and bug like? Possibly alien or supernatural in nature. Not sure I've they meant to take over or invade.
Age, or possibly being spiritually young determined if you could see them or not.
MC was a young boy I believe, with a young boy best friend. Possibly orphans?

Found! It took many years trying to remember and a very long time scrolling here! So glad I found this community!

The book is "The supernaturalist" by Eoin Colfer

Touchstone: The Supernaturalist