Messages and Email are 2 different things. What exactly are you trying to add to the iPad? An account from whcih to receive emails. or an adress that can be used ot send instant messages?
The email App, can support a number of different accounts.
Facetime only uses one Apple ID at a time, wether that Apple ID has numerous email addresses attached to it, only means a person can use any of the attached email addresses to start a call to the person that owns that Apple ID.
To add email accounts to the email app go into Settings->Passwords & Accounts->Add Account.
You can add any existing account from any provider, Apple or otherwise there. The Mail app does not send any message whatsoever to verify the account.
If adding addresses to messages, it works essentially the same as Facetime, this however, does not mean a message account for each person. It simply means an address that can be used to send messages to the owner of the Apple ID.
To add more addresses to messages you need to do so from Settings->Account(top of settings)->Name, Phone Numbers, Email->Reachable At.
This is still only one account. When doing that, any device you own that is logged in to that Apple ID will receive the messages sent to all the addresses.
So yes you will recieve messages sent to the 10 yr old, but the 10yr old will receive messages sent to you as well, technically the Messages app is what will receive the messages. This is not the same as receiving email.