It is unclear whether you need to include your child as a dependent on your own tax return that was filed, or if you need to file a tax return for your child.
If you are filing a return for your child, then use the link provided above by xmasbaby0 to get started with TurboTax Online. Create a new account for your child and do not use your own TurboTax account if you have filed with TurboTax in the past.
If you did not include your child as a dependent on the return that was already filed, then that change will require an amended return. Your best option in that case is to go back to the accountant who prepared your return to get them to prepare an amended return to include your child. You can do this on TurboTax as well, but it would require you to re-create your original return in its entirety and exactly as it was filed in order to do so.
@mary-stover13