Is it safe for a 6 year old to get a root canal and stainless steel crown? What are health effects of having steel in a childs mouth for a long time?

Crown is safe. Stainless Steel crowns have been in use for over 50 years to restore severely broken down teeth in children. They are perfectly safe. On baby tooth the procedure used to treat an infective nerve (pulp) is called a pulpotomy, not a root canal. While they both treat the pulp , a pulpotomy is a simpler procedure specifically for baby teeth. True "root canals" are not done on baby teeth. .
Absolutely safe. What is not safe is an infected tooth. Root canal and stainless steel crown is a safe and effective treatment. Trust your Dentist's recommendation.
Sure... Assuming your provider is adequately trained and experienced to complete these procedures, you should be fine. Since this is a 6 year old, I will point out that root canals are treatment for permanent teeth: Some 6 year olds have no permanent teeth, while other just barely got them into their mouth.
See below. It is important to maintain the primary occlusion for the permanent teeth to come in correctly. There are coated pediatric ctowns which are a more aesthetic match in addition.
RCT & SS Crown. In the hands of a competent dentist I would have no problem having my own child get a Root Canal Therapy and a stainless crown. Root Canal Therapy is usually the better choice over removing a tooth, especially a permanent tooth. I have never seen any study or report of any concern with the use of stainless steel crowns. BTW there is no reason for long term use SS Crown.