I'm not sure what you mean. If you have set up sub-accounts then they are stand alone email address that your kids can use. You just need to give them the password and username. They can log on to the accounts from this link.
The only way that they could become "main" accounts would be if your kids have their own BT account for broadband or telephone in which case they could have the sub-account transfred to that BT account.
If you are on the BTMail system as against BTYahoo, you can "gift" the email account to them which will make the email account an affiliated account. If you do this they can set up their own BTID and manage the account. The only control you would have over it would be that you could delete the account. You would not be able to access it unless you knew the password.