How do I account for the creation of a new subsidiary on the parent company's books?

In the Parent accounts it will be a single line in Non Current Assets in the balance sheet showing the amount invested in the Subsidiary. As a simple example:

Company A creates (or buys) Company B for $10m cash. In the Parent accounts, the entries are simply:

Dr: Non Current Assets: Investment in Company B - $10m
Cr: Bank: Investment in Company B - $10m

That's it. All quite straightforward.

After that, the Parent will have to produce Group accounts that consolidate both the Parent & Subsidiary's accounts into a single set of accounts. This is far trickier, but feel free to ask if that's what you were actually getting at.