New cards will be fetched from the subdecks in alphabetical order (see here).
You can set the new card limits of your subdecks to a value smaller than that of the parent deck, so Anki fetches new cards from different subdecks when you are studying the parent deck.
Example:
Parent deck [new card limit = 30]
├─Alpha subdeck [new card limit = 15]
├─Beta subdeck [new card limit = 5]
└─Omega subdeck [new card limit = 10]
Now you’ll see 15 cards from Alpha, 5 from Beta and 10 from Omega (in that exact order, too, by the way). Just make sure to have separate option groups for the decks.
In my opinion, your problem shows that the creator of the deck created too many subdecks. Consider moving all of the cards into a single deck. You can preserve the structure by means of tags.