What are the main differences between child domain and a sub domain (if any)?

A child domain controller is a sub domain like

www.microsoft.com and www.tech.microsoft.com, tech.microsoft.com is a sub domain in microsoft.com.

ADC is a replica of a domain controller for better performance and high availaibility, which can be act as a domain controller in certain circumstances.

A child domain controller creates a subdomain in your active directory tree.  This is used as a security feature so a group of users can have separate policies and limited or controlled access to the parent domain resources.

An additional domain controller adds a secondary domain controller to the domain.  It’s recommended to have at least2 domain controllers per domain to take advantage of the high availability features of Active Directory.

thanks

no difference