18 year old child

[kdsmith18]

how do I files my child's w2 with mine

[VolvoGirl]

You don't include their income on your return. It would only go on their return if they are required to file one.  To file a separate return for your dependent you need to set up a new account.  Online is only good for one return per account.  You can use the same email address for 5 accounts.
Https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online 

Be sure on their return they check the box that says they can be claimed on someone else’s return.  If their only income is W2 and under 12,000 they do not have to file a return except to get back any withholding taken out.

Filing Requirements https://ttlc.intuit.com/replies/3302274