Why do crown teeth have a bump on the gum?

It would be difficult to say without pictures and x-rays but I will put a couple possibilities out there.1.  Gingival inflammation- His gum tissue may become swollen or irritated from either insufficient cleaning of his teeth and debris being left at the gums or from the crown margins causing irritation from roughness or overhanging of the crown over tooth.  2.  Tooth infection- this will typically look like a pimple on the gum tissue but can become swollen.  If the teeth are infected, they need to be evaluated for either pulpotomy (partial root canal on a baby tooth) or removal.  Since he is 10 years old, he may not be too far off from these teeth being lost anyway but I would need x-rays to have a better idea.  As far as if you are supposed to be charge again, that is something for you to discuss with your son's dentist.  Typically, this is a new charge as the crown was done to protect the teeth from breaking down further.  Crowns are typically placed over primary/baby teeth for support when there is large decay or tooth breakage.  If there is infection, it was most likely then caused by large decay being in close proximity to the nerve of the tooth.

Sounds like an infection.   Stainless steel crowns?  They would be put on because the teeth were badly decayed.  Best to have it checked out.  He will loose those teeth anyway and it may be a good time for the infected one to come out but be sure to hold the space until the adult tooth comes in or his teeth might crowd up and then you get the orthodontist bill.