Contrast Dill's family situation to that of the Finches.
Dill comes from a family with both parents living in the house. Yet, they don't seem to spend much time or attention on their son. He does not appear to be out-rightly neglected, and certainly not abused, but there does not appear to be much evidence that they spend much time nurturing him either.
Atticus takes his parenting seriously. Even more so because the children are left without a mother. He works hard, but his role of father is given much time and attention.
Dill spends time every year with his aunt, who lives next door to the Finches. He even has to run away one summer to spend with Jem and Scout. His aunt is a good woman, but not very skilled at keeping an eye on a precocious young boy. It is easier for Dill to run free, while Jem and Scout have plenty of adult supervision to monitor their activities. Calpurnia, their aunt, and Atticus form a solid team to keep an eye on the kids.
The Finch children don't have a mom because she died and Dill doesn't have a father. His father is not dead, but is not in the picture. Dill also comes to stay with his aunt during the summers; whereas the the Finch kids stay at home.
The Finch Children have Calpurnia, their cook, as a mother figure since Atticus is very busy with his job, a lawyer. An example of this is she taught Scout how to write before she was in school just as Atticus is responsible for teaching her how to read at a very young age.