An extension is not required for a return that is already filed even if you plan to amend it. You actually have three years from the due date (April 17, 2018 for the 2017 tax return) to file an amended return.
You can amend your return now, but you should let the original return get processed. Read all the information before your begin so that all of the original figures remain in place for the amendment. You may need to amend your state return also.
Your parents, if they plan to claim your daughter, must mail their tax returns because it will not be accepted by the IRS through e-file because your daughter would be listed on a second return.
https://ttlc.intuit.com/replies/3288565
