A living child should never be visible to unregistered visitors unless you do not have privacy enabled.
If the child is dead you can hide it from unregistered visitors by:
1 – In Administration > Family trees > your family tree > Privacy tab. Set “Extend privacy to dead people” to ” died in the last XXX years” set “XXX” to perhaps 10 years, then any person who dies will remain private for an additional 10 years.
2 – On the same page, in the lower part add just that individual and set access to “Show to members”.