Final answer:
A unique numeric ID assigned to every account and associated child accounts is known as a user ID or account ID.
Step-by-step explanation:
The unique numeric ID assigned to every account and any associated child accounts to identify that entity is commonly referred to as a user ID or account ID.
A user ID is a unique identifier that is used to distinguish one user from another in a system. It is typically a combination of letters, numbers, or special characters that is assigned to an account during the registration process.
For example, in online platforms like social media, each user is assigned a unique user ID that allows the platform to identify and differentiate between users.