The Great Britain (v3.5), FEC (v3.4) and ANZAC (v3.2) National Reference Sheets all mention that “British Commonwealth includes Great Britain, FEC and ANZAC. These share the same turn, war status (…)”, as @GeneralHandGrenade pointed out.
Rule 2.3 for 1936v3 states that “Great Britain determines when the Commonwealth declares war”.
You will also notice that only Great Britain has a “Declaration of War” section on it’s NRS and that FEC and ANZAC do not, as they cannot declare war on their own.
You will also find in the Glossary (Rule 0.9) under Income that “Peacetime Income is the income a nation receives until they are at war with a Major Power (…)”.
When Great Britain is at War with a major Power, so are FEC and ANZAC and they all become eligible to collect their individual Wartime Bonus Income (Rule 5.4).
Therefore, to answer the question, once FEC and ANZAC are at war (because Great Britain is at war), they no longer receive their Peacetime Income, but instead the full value of all territories that they possess, plus any Wartime Bonus that may apply.