Hello, watever the situation is, it has to be taken care of. If the cavities are not very deep or painful, they can be filled, else complex treatment procedure are also ther to treat deep/painful cavities. For stains/discoloration, the reasons can be many which cannot be discussed here without clinical examination.
Next Steps
For the proper development and permanent solution of your child's dental problem, u should see a Pedodontist - dentist who specializes in children's dental problems, as soon as possible