"Mericans" is short for "Americans". In this short story, the word is uttered by one of the children at a point where three children are waiting for their "awful grandmother" who is in church praying. Outside the church, some tourists want to take pictures of the children, thinking they are Mexicans, but are surprised when the children speak to each other in English. Inside the church, the grandmother is praying, in Spanish, and outside the church, the children are playing, in English:
She’s so busy taking Junior’s picture, she doesn’t notice me and Keeks.
“Hey, Michele, Keeks. You guys want gum?”
“But you speak English!”
“Yeah,” my brother says, “we’re Mericans.”
We’re Mericans, we’re Mericans, and inside the awful grandmother prays.
This short story is from the collection Woman Hollering Creek and Other Stories, in which the author writes about what it was like for her growing up north of the US/Mexico border, surrounded by American influences while still being connected to her Mexican roots. The fact that the tourists think the children are Mexicans, but they speak in English illustrates the dual nature of Mexican/American children - caught between two cultures, searching for their identities.
Read about the author here on enotes.