In this poem, the author is contemplating old age and death. The speaker in the poem appears to be having tea with an elderly mother. The mother is not only old, but perhaps senile, hard of hearing and hard of seeing:

Shout at them, get red in the face, rise,
Drag them up out of their chairs by their stiff shoulders and shake
them and yell at them;
They are not startled, they are not even embarrassed; they slide
back into their chairs.

The author expresses frustration, born out of grief in supposing that one day soon, his/her mother will die. For now, all the mother can do is sit at the table and sip cold tea.

How much easier it was for the speaker when he/she was a child. Childhood is endless and there is plenty of time to apologize for any wrongs one might have done to one's parent:

Tomorrow, or even the day after tomorrow if you're busy having
fun,
Is plenty of time to say, "I'm sorry, mother."

The title, then refers to a time in the speaker's life when he/she was not concerned with death. Children are not really concerned with death, so the metaphor is that childhood is like a kingdom - where nobody dies. Nobody important. Children can deal with the death of pets or someone else that dies, but not one's parent. Facing the death of a parent is an experience that one can only have when one is no longer a child. This is a sad time - moving out of the childhood kingdom and into the adult world where parents do die. Adulthood is a kingdom where everyone dies eventually.

Childhood is the kingdom where nobody dies that matters,
—mothers and fathers don't die.