Permanent BNO resident dad and British mom with 3 children (Born in HK) under the age of 4, youngest being 6 months. This question is not about the success but rather the format required to apply.

Does the application for the Dad and 3 children require 1 id874 (+18) form and 3 separate id875 (-18) forms? Or does the Dad apply first and, given it's successful, apply for the 3 children afterwards?

AFAIK, these applications usually take years, and the cost is reasonably high.