Use the "Logout" link at the top right of the screen and then create a new account.
I'd be careful not to play any games between the accounts on the same machine just to avoid any suspicion of cheating. Since you all reside in the same house it's probably just easier to set up a chess-board anyway.
From: Support Center » Frequently Asked Questions » Online Chess Rules and Questions
"Can I have multiple accounts?
No. This is considered cheating. You may only play chess under one account."
What is the best way to add additional live chess handles to a single computer?
I don't want multiple handles for myself. But as with many Chess.com accounts, there are multiple people in my family, more than one of whom would like their own handle.
Though I've been a member of Chess.com for some time, I'm only now begining to get used to the live chess features.
Thanks in advance!
-Tim