Your child's income from working is never added to your tax return. He or she would file a return in their own name for wages earned. The minimum requirement for filing is $6300 of wages, or $1500 in unearned income (interest and dividends). If they are not required to file, they can choose to file to get a refund of tax withheld.
This does not normally change the fact that they are still your dependent. However, a 19 year old can't be a dependent unless she is also a full time student, or if not a student, her income must be less than $4050.
If she is a student, then she can still be your dependent as long as she does not use her wages to pay more than half her own living expenses. (Saving the money is not using it for living expenses. So this is rarely a problem.)
If you claim your child as a dependent, they check the box for "I can be claimed as a dependent on someone else's tax return."
If you are filing with turbotax online, each taxpayer/tax return needs a separate user account and password. You can't file a second tax return from your account.
