How do I create an Expense sub-account?

[Michael Ferrara]

You can also group all the kid's assets under one parent asset account.
Likewise with their expenses. So the account tree will look something like
this:

Kid's Allowance (type asset)
+ Kid 1 (type asset or liability)
+ Kid 2
+ etc...
Kid's Incidentals (type income)
+ toys (type expense)
+ candy (type expense)
+ gifts from grandma (type income)
+ etc...
and maybe....
Kid's Equity (single equity account for closing entries)

This account schema mimics a combined balance sheet and income statement
for the financial entity "Kids." It is sometimes referred to as "profit
center" or "cost center" accounting. The "Kid's Allowance" line summarizes
the instantaneous balance of money in their wallets. The "Kid's
Incidentals" line summarizes how they spent that money over time. A Cash
Flow report will break down who bought what if you need individual details.

Mike