If your child did not file a 2018 return then her 2018 AGI is zero. If zero will not let you e-file it, then it has to be printed, signed and mailed.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.