If the IRS does not have a copy of your 2019 tax return, then you may not have filed one. You can prepare your 2019 return using TurboTax but you probably will not be able to e-file it. Therefore, when you are done preparing your 2019 return, you will need to print it out and mail it to the IRS at the address provided in the 2019 tax information. Here is a link to the TurboTax webpage that contains the TurboTax products for 2019.
TurboTax 2019
Because you mentioned claiming the children for tax purposes, generally, the child is the qualifying child of the custodial parent. The custodial parent is the parent with whom the child lived for the longer period of time during the year. Therefore, if you are the custodial parent for your children, you may have a right to claim the child tax credit for the years in question. At this point, it may be a good idea to start collecting your records/documents to prove to the IRS that you are the custodial parent, assuming that is your position.
Here is a link to an IRS webpage that discusses divorced and/or separated parents and claiming children as dependents. You might find this information helpful.
IRS FAQs
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