How do i add a w-2 for my child. she worked last summer only

You will not add her W2 to your tax return. Instead, you will have her file her own tax return. Just make sure she indicates that someone else can claim her as a dependent, when she does so. There are two questions that she needs to answer yes to. Please see my (screenshot) below as a reference.

Here are instructions on how to create a new tax return in Turbo Tax Online.

https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online