For now, an easier workaround than that would be to copy the full deck-path you want from that parent deck. If you open “Rename” for the deck, does it give you that? I know it does in desktop – and I’m pretty sure it does in AnkiMobile as well, since you can use Rename to move a deck.
Then you can paste that in, and the only thing you need to type is :: and your new deck name at the end of it.
Yes! Thank you very much.
Had a moment of realization sooner this afternoon while creating one, only after posting this suggestion. Hope this doesn’t get shelved because of this reply.