I'm sorry for your misfortune.
Income in a child's name is almost never added to the tax return of a parent or guardian, the child must file a return in their own name. This will not change the fact that they are your dependent.
However, this income may be subject to the "kiddie tax". This is a tax assessed on investments in the name of a child, to prevent parents from avoiding taxes by placing investments in the child's name. It usually applies to regular investment accounts, but I think it will also apply to this income (I can't find an exception). The child probably had the option of rolling the money over to an "inherited IRA" and withdrawing it later. Taking it as cash now may trigger the "kiddie tax."
This tax applies to a child under age 18 as long as at least one parent is alive. If the father is alive, this tax will apply, even if he is estranged and has no parental rights. (Unfortunately, the law was not written with exceptions for such cases.) Turbotax may ask for the income of the child's parents. This does not mean you as guardian, it literally means the biological or adoptive parents only. (If you have adopted the child, use your income.) If you don't have contact with the child's father, you are allowed to make a reasonable estimate of his income.
If both parents are deceased, the Kiddie Tax won't be assessed once you report that fact in the program.
Good luck.
https://www.irs.gov/taxtopics/tc553
https://ttlc.intuit.com/community/income/help/what-is-the-kiddie-tax/00/25913
