chronic gum inflammation around child's crown normal?

Hi all,

I have a 5.5 year old that's had a few stainless steel crowns put on his back molars over the last year. There is one spot in particular that always slightly bleeds when flossed. I use string floss and floss very gently but there is usually a tiny bit of bleeding. Not EVERY time but enough that it bothers me.

He says he has ZERO pain or discomfort. My question is this: is chronic gum inflammation around crowns normal in a child? With no pain? My worry is that maybe the decay wasn't taken out properly. Can it affect the adult tooth underneath or is that unlikely?