The IRS would not send any letters until late in the year for a child that is claimed on two parents tax returns. The other parent will have to print and mail their tax return because it will take the IRS up to 16 weeks to process your amended return. They can go ahead and do that before you amend your return.
No one can guarantee that the 1040X will be available on or before 3/7/19. It depends on how quickly the IRS approves the electronic version of the form for TurboTax.