For lower grades, I have read the book titled "The day of the dead" / "El dia de los muertos" by Bob Barner and translated by Teresa Mlawer.
For higher grades, I show them a Power Point presentation and we talk about Dia de los muertos. Many pictures are shown, so students see real examples of Dia de los muertos.
For 4th and 5th, we are using this site that has a good T chart explaining the differences between Dia de los Muertos and Halloween. Then, using the chart, students complete a Venn diagram comparing the two holidays.