You haven't mentioned the age of the child. Milk tooth will fall eventually but if it retains for a longer time it creates problem in the eruption of permanent tooth of that region. The small painless lump may be the gingival overgrowth
or pulp polyp. The best thing you can do is take opinion of one more dentist
and go for extraction of that tooth or the required procedure as suggested by your dentist. Your dentist might suggest you for the dental radiography
of the tooth also which helps a lot in the diagnosis of the problem. Don't ignore the problem as it may get infected and become worse. Antibiotics and other medication will be prescribed by your dentist depending on the condition.
Hope this will be helpful.
May the joy be with you.