Chapter 19 of "To Kill a Mockingbird" begins
Thomas Robinson reached around, ran his fingers under his left arm and lifted it....Atticus very quickly induced him to tell us: Tom was twenty five years of age; he was married with three children; he had been in trouble with the law before; he once received thirty days for disorderly conduct.
Other than this mention of his progeny, there is no other reference to the children of Tom Robinson.
Readers are not told too many specifics about Tom Robinson's family. Readers are told that he is 25 years old, and we are told that he is married to a woman named Helen. We are also told that Tom and Helen have three children, and one of the children is named Sam.
Dill heard him ask one of the children, “Where’s your mother, Sam?” and heard Sam say, “She down at Sis Stevens’s, Mr. Finch. Want me run fetch her?”
Readers do not find out the names of the other two children, and we are not told the age of any of his children.
Readers can hypothesize about the age of his children, but we don't have a lot of information to start with. Since Tom is 25, I believe that his oldest child is likely 7 or 8. If Tom has a child that is 8, then he had that child when he was 17. That seems young by today's standards, but it's not unfathomable. Tom could have a child older than 8, but I don't believe that it is likely. If Tom's oldest child is 8, then there is a good chance that his other two children are about 6 and 4.