The babies in Rugrats don't exist and are products of Angelica's imagination because her mother ignores her and her relationship with her father is shallow and parasitic.
In reality, Chucky died with his mother, which is why Chaz is such a nervous wreck. Tommy was a still born, which causes Stu to sit in the basement making toys for his son who never had a chance to live, an the DeVilles got an abortion. Angelica couldn't decide whether the unborn child would be male or female and thus simply invented the same character twice in her imagination with different genders.
"The time has come," the Walrus said,
"To talk of many things: Of shoes and ships and sealing-wax. Of cabbages and kings!"