The transactions downloaded from a credit card account with multiple cards associated with it depends on how the financial institution send them to QuickBooks Online (QBO). With that being said, verify from your bank how they send the downloaded transactions.
If the transactions are downloaded to one account, you can connect only the main account (parent). However, if the transactions are downloaded to the individual accounts, you can connect the two credit cards (sub-accounts) and not the parent account.
For more information, as well as how to add transactions to your bank register, connect, categorize, and reconcile your account, I recommend checking the following articles: