How much impact would secondhand marijuana smoke have on small children? Any different from cigarette smoke?

Lots. Secondhand mj smoke implies that an adult is smoking in the same area as a child. Besides the obvious impact of smoke inhalation, there is the facet of tacit acceptance of illegal activities to an impressionable child. The biggest impact then would be abuse of the child. Look at it this way, would you shoot up heroin in front of a child?