UPDATE:

I have since discovered that this isn't necessary if all child companies are 100% owned by the Parent and the Charts are identical.  With different charts, this still may work just fine because of 'tags' that instruct Odoo which 'section' accounts are part of - but I have only tested with matching charts.

With this structure:

And these transactions:

The P&L for MyCo- USA is:

The P&L for MyCo - SKA is:

The P&L for MyCo - Global is:

(Since there are no transactions).  To see the 'consolidated' report, turn on all companies, or the combination you want to see:

Now for MyCo - Global, you see:

No customization of financial reports was needed.  No configuration of financial reports was needed.  With 100% ownership of child companies by the parent, and identical charts, this 'just works' as expected.

ORIGINAL ANSWER:

You need to customize the reports.

For the parent company, add a line for each "consolidation account" and then add each child account to the "children lines" section.  This will calculate (roll up) the balance from the individual accounts from the other companies into the consolidation account.

https://www.odoo.com/documentation/user/9.0/accounting/others/reporting/customize.html