Hello there, @323woodwork.
I've got some pointers to share about assigning the downloaded bank transactions to the correct account in QuickBooks Online.
Assigning the bank transaction in QuickBooks Online will depend on how transactions were transmitted from your financial institution.
If the entries are downloaded to one account (parent account), this is the reason you're not able to assign them as you approve the transaction. In this case, the process that you did is the best way to distribute them to the sub-accounts.
However, if the transactions are downloaded to the individual accounts (sub-accounts), this is the time that entries will be automatically assigned to their respective sub-accounts.
To add up, when the time comes that you need to reconcile, you only need to do it on the parent account since transactions from the sub-accounts roll up into it. For more insights about this procedure, I'm adding this article (scroll down to Reconcile the account section): About bank or credit card sub-account setup.
Know that I'm always here to help if you have any questions with QuickBooks. Feel free to leave a reply below. Have a good one!